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Adult Wistar rats (Rattus norvegicus), Apodemus agrarius, Meriones unguiculatus, Clethrionomys rufocanus, and Apodemus argenteus were inoculated with Rattus-type (SR-11, KI-262, and TB-314) or Apodemus-type (Hantaan 76-118) hantaviruses. Production of serum antibody to the inoculated virus (IAHA titres of 1:32 to 1:4 096) was obsersved in all rodent species 10 weeks after virus inoculation. Rattus-type virus was detected in some organs of all the rodent species employed except of Apodemus agrarius. Apodemus-type virus was found only in some organs of Apodemus agrarius. Newborn Wistar rats induced antibody in high titres to both Rattus- and Apodemus-type hantaviruses. Rattus-type virus was detected in all the organs examined for up to 6 weeks after inoculation, whereas Apodemus-type virus disappeared from all organs except of brain and lung tissues. The virulence of the three Rattus-type viruses to newborn rats was different. These findings indicate that the susceptibility of rodents may vary depending on the combination of rodent species and virus strains; they also suggest that the various species of rodents may be the reservoir animals of hantavirus infection in nature.

The case of a 53-year-old female with interstitial pneumonitis is described with special regard to biochemical characterization of pulmonary corpora amylacea which were found in the lung specimen obtained by bronchial biopsy from the patient. The main protein component in bronchoalveolar lavage (BAL) fluid of the patient was albumin, but proteins in the precipitate fraction of BAL fluid, where the corpora amylacea were recovered, predominantly consisted of 36 kD protein which was stained with the monoclonal antibody PE 10 to human pulmonary surfactant apoprotein by immunoblot. Histologically the pulmonary corpora amylacea were stained with eosin and PAS. The particles were stained immunohistochemically by immunoperoxidase reaction using PE 10, but not by antibodies to human albumin. The pulmonary surfactant apoprotein seems, therefore, to be not simply adsorbed in the particles, but to be contained in them. Thus, the surfactant apoprotein may, at least in this case, be involved in the formation of pulmonary corpora amylacea.

The LEC rat, which suffers from hereditary hepatitis, was examined for elucidation of its clinicopathological characteristics during development of the acute phase of hepatitis by quantitative analyses of histological observations of the liver in combination with laboratory data on various serum enzymes. The progression of acute hepatitis in the LEC rat was observed to begin insidiously early in life, i.e., a few enlarged hepatocytes and Councilman bodies appeared at around 8 weeks of age without clinical signs. Furthermore, it was revealed that the acute phase of hepatitis started with a remarkable increase of Councilman bodies, large nuclei and hepatocytes in mitosis in the liver 3 to 4 weeks before the onset of fulminant hepatitis, which is characterized by the elevation of serum enzyme activities such as GOT, GPT and gamma-GTP, and the onset of jaundice. From those observations, three stages were proposed for the progression of acute hepatitis in the LEC rat.

A sensitive enzyme immunoassay method for rat gamma-glutamyl transpeptidase was developed that was able to quantitate between 5 to 500 ng of the enzyme. The enzyme contents of fetal liver, adult liver, hyperplastic nodules, and various tissues including hepatoma were determined. Normal rat liver contained very little enzyme, while fetal liver and hepatoma tissue contained high levels. The gamma-glutamyl transpeptidase content of the liver was assayed during the process of 3'-methyl 4-dimethyl-aminoazobenzene-induced hepatocarcinogenesis. The content was increased at an early stage, then decreased, and again increased at the stage when hepatoma developed. The change of the enzyme content determined by an enzyme immunoassay was highly consistent with that determined by enzymatic assay. This indicates that the increased activity of the enzyme as previously reported was accompanied by an increased immunoreactive enzyme protein. Immunohistochemical studies on the hyperplastic nodule revealed the presence of gamma-glutamyl transpeptidase in the hepatocyte membrane.

Five distinctly different types of naphthyl acetate esterase in rat liver were examined for study of liver enzymes during hepatocarcinogenesis. Three types of esterase in normal adult liver were separated by column chromatography. Main esterase in adult hepatocytes, which was demonstrated near the origin in cellulose acetate electrophoresis, was very sensitive to diisopropyl phosphorofluoridate (DFP). The other two esterases showed different electrophoretic mobility, while their Km values did not differ and both were considerably resistant to DFP. An anodic minor component in normal adult liver, which had a characteristic esterase pattern of infant liver, increased in the liver of rats fed 3'-methyl-4-(dimethylamino) azobenzene. This esterase was obtained by electrophoresis on Cellogel block. It was partially inhibited by DFP and p-chloromercuribenzoate, activated by cysteine, and showed a different Km value from the above esterases. Another minor component situated at the most cathodic side, which had characteristic esterase patterns of fetal liver and hepatoma, was very sensitive to DFP and eserine, and showed a characteristic of nonspecific cholinesterase as proved by staining.

Alpha-fetoprotein (AFP) in the sera of 35 LEC (Long-Evans with a cinnamon-like coat color) rats between 7 and 25 weeks of age was evaluated by enzyme-linked immunosorbent assay (ELISA). Elevation of serum AFP and proliferation of oval cells in the liver were observed in most LEC rats, which suffered from acute hepatitis. On the other hand, the serum AFP level was within the normal range before the onset of hepatitis. Immunohistochemical staining for AFP revealed that some of the proliferating oval cells produced AFP. Morphometric analysis of AFP-positive cells and ELISA for serum AFP demonstrated that there was a statistically significant correlation between the number of AFP-positive cells in the liver and the concentration of AFP in the serum. Histological examination revealed the transition and differentiation of the oval cells to small hepatocytes. These results suggested that the phenomena which occurred in LEC rats suffering from acute hepatitis were similar to those that occurred during the early stage of azo dye hepatocarcinogenesis, although the extent of the oval cell proliferation and the elevation of serum AFP in LEC rats were not as great as those in rats treated with azo dye. This is the first report on a rat strain with proliferation of AFP-producing oval cells during its natural history.

Spontaneous hepatitis associated with severe jaundice occurred in 90% of an inbred strain of Long-Evans rats. The rapidly progressive syndrome was characterized by abrupt onset, hyperbilirubinemia and increased serum levels of glutamic oxaloacetic transaminase and glutamic pyruvic transaminase, associated with massive and multifocal necrosis of the liver. This strain should provide a useful animal model for analysis of the pathogenesis of fulminant hepatitis in humans.

We investigated LEC rats immunopathologically which spontaneously developed hepatitis to find out the genesis, in comparison with non-hepatitis LEA (Long Evans Agouti) rats. 1) Wet weights of the spleen and thymus of 6-week old LEC rats were significantly lighter than those of LEA rats of the same age. 2) Serum IgG (Immunoglobulin G) in LEC rats remained markedly low after the age of two months and IgG antibody formation to SRBC (Sheep Red Blood Cell) as detected by plaque assay was also significantly suppressed. On the other hand, IgM antibody formation to SRBC was significantly suppressed through serum IgM level in LEC rats was normal or rather increased. 3) Blastogenic responses of spleen cells to PHA and Con A were much more suppressed in LEC rats than in LEA rats. 4) Cytostatic activity of intraperitoneal macrophages against tumor cells was more evident in LEC rats than in LEA rats, but there was no difference in NK (natural killer) activity between the two rat strains. From these results, it is speculated that spontaneously hepatitis-developing LEC rats possess T and B cell deficiency (combined immunodeficiency) and that the increase of macrophage and NK cell activities are linked to the genesis of developing hepatitis.

At the Center for Experimental Plants and Animals, Hokkaido University, two inbred strains, Long Evans Cinnamon (LEC) and Long Evans Agouti (LEA), which were selected for coat colour, were isolated from a closed colony of Long Evans rats. While the two inbred strains were maintained by sibmating, only LEC rats, over 24-generation, spontaneously developed acute hepatitis with sudden appearance of systemic jaundice at around four months after birth. The frequency of acute hepatitis was 80 to 90% and the disease in nearly 80% of these rats were progressive and they died within two weeks after onset, with their clinical course and histopathological findings similar to those of human fulminant hepatitis. LEC rats with spontaneous hepatitis had strong-conversion of urine-bilirubin, ultimate increase of blood-bilirubin and abnormal increase of serum-transaminases (GOT, GPT; GOT greater than GPT). Histopathological findings of the livers in the rats with acute hepatitis showed spotty necrosis and abnormal hepatocytes containing giant bizarre nuclei and in the rats with fulminant-type hepatitis showed central or coagulated necrosis and marked infiltration of inflammatory cells. Serum levels of albumin in LEC rats before being affected by hepatitis were low compared with those of LEA rats and especially characteristic fact was that cellulose-acetate electrophoresis could not reveal gamma-globulin fraction in LEC rats of 6-week and 12-week old, which will be a clue to analyze the etiology of hepatitis in LEC rats.

The LEC (Long-Evans with a cinnamon-like coat color) rat is a new mutant strain with hereditary hepatitis. The rate of DNA synthesis, the relative amounts of binucleated cells, and the polyploidizations of LEC hepatocytes have been analyzed. Markedly high polyploidy, such as 32n and 64n, were detected after manifestation of hepatitis; however, no aneuploidy was detected. Bi-, tri- and tetranucleated cells whose nuclei occasionally differed in size were observed after the manifestation of hepatitis. In addition to small hepatocytes and oval cells in the periportal area of the hepatic lobule, enlarged cells with huge nuclei were also labeled with BrdU, indicating that in LEC rats suffering from hepatitis abnormal mitosis may be relevant to high polyploidization and multinucleation. Polypeptide analysis using 2D-PAGE detected the apparent lack of expression of two polypeptides, p29.5 and p30, in LEC liver cells; however, linkage analysis indicated no correlation between these peptide defects and the manifestation of hepatitis.

Induction of a microsomal Ca2+-dependent serine protease by hepatic tumor promoters was studied. Male F344 rats were fed a diet containing one of the following promoting agents: phenobarbital (CAS: 50-06-6), dichlorophenyltrichloroethane (CAS: 50-29-3), butylated hydroxytoluene, ethyl-alpha-chlorophenoxyisobutyrate (CAS: 128-95-0), or 17-alpha-ethynylestradiol (CAS: 57-63-6) or a nonpromoting agent, diphenylhydantoin (CAS: 57-41-0), for 1 week. By treatment with promoters, the protease activity in the microsomal fraction was increased to threefold to fivefold that of control, whereas only a slight increase of activity was found after diphenylhydantoin treatment. The Ca2+-dependent protease activity was determined with the use of N-benzoyl-L-tyrosine ethyl ester as the substrate in a medium containing 50 mM CaCl2 for its maximal activity. This protease was preferentially localized in the smooth microsomal membrane and strongly inhibited by diisopropyl phosphorofluoridate (CAS: 55-91-4), and the optimum pH of the activity was 7.8. It appears that the Ca2+-dependent serine protease measured by using a chymotrypsin substrate is a novel protease, and induction of its activity by hepatic tumor-promoting agents is a common and specific phenomenon.

1,2-Dimethylhydrazine dihydrochloride (DMH) is one of the most reliable carcinogens for experimental colonic carcinogenesis. In order to evaluate the biological significances of target organ specificity of DMH-induced carcinogenesis, specific pathogen free (SPF) BALB/c mice were injected subcutaneously with DMH for 30 weeks, and were studied pathologically for tumor development of various organs. The data showed that colon tumor developed only at 2% of these mice. However, it produced 100% incidence of malignant endotheliomas and angiosarcomas of blood vessels and these tumors selectively appeared in the liver. There were no overt foci of primary vascular tumors in the heart, lung, kidney, muscle as well as spleen. This tumor model may reveal that the organ specificity of the carcinogenic activity with DMH could be converted, possibly depending upon the bacterial flora in the intestines, and very suitable for the investigation of implications of intestinal bacteria on the colonic and vascular tumorigenesis.

The study was carried out to identify alpha-fetoprotein (AFP)-producing cells in the hepatic tissue by immunofluorescent antibody techniques during the early stage of 3'-methyl-4-dimethylaminoazobenzene ingestion. After 1 to 3 weeks, cells fluorescent to AFP were undetectable in cholangiolar cells ("oval cells") and also in degenerated megalocytic hepatocytes. After 4 to 7 weeks AFP appeared in rat sera, and "transitional cells" and small hepatocytes proliferated markedly in the periportal areas of hepatic lobules. AFP was exclusively detected in the majority of the transitional cells and a small portion of the small hepatocytes. Some fluorescent cells appeared in small groups, and others were randomly distributed in the periportal areas. The typical oval cells and the megalocytic hepatocytes were not fluorescent. When AFP in sera became undetectable, the regenerated hepatocytes matured considerably and were not brightly fluorescent. In the hepatic tissue, where AFP-producing cells were observed by fluorescent antibody technique, hematopoietic cells were frequently observed but they were not fluorescent.

A microsomal butyrylesterase (L-I) was purified from the livers of male W rats treated with phenobarbital, and an antiserum against this purified L-I was raised in a rabbit. By the Ouchteriony double-diffusion test, a precipitin line was observed between the anti-L-I antiserum and each Triton X-100 extract of livers during development, regeneration after partial hepatectomy, and carcinogenesis and of hyperplastic nodules and hepatomas, all of which revealed L-I in their esterase isoenzyme patterns. These precipitin lines exhibited esterase activity. The fusion of the lines of these tissue extracts and that of the purified L-I indicated the presence of an antigen site common to their esterases. The extracts of adult and fetal livers and also of hepatomas resembling fetal liver in the esterase isoenzyme pattern did not produce a precipitin line with anti-L-I antiserum.

A quantitative analysis was performed on the extramedullary hematopoietic foci which appeared in the liver during the early stages of hepatocarcinogenesis with 3'-methyl-4-(dimethylamino)azobenzene in rats. The frequency in the appearance of the foci reached a maximum at around 3 weeks of azo-dye feeding. Since the liver in this period has been known to show deviation of various characteristics toward fetal liver, an intimate correlation of the appearance of the foci and fetal character of the liver was suggested. Histologically the hematopoietic foci were always observed in the sinusoidal space of original hepatocytes adjacent to the oval cell proliferating area. Ultrastructurally the cells of the foci were identified as erythroblasts and were found in the space of Disse as seen in the hematopoiesis in fetal liver.

Spontaneous occurrence of placental glutathione S-transferase (GST-P)-positive foci was observed in the livers of 5-month-old LEC rats. Quantitative studies revealed that GST-P foci appeared after the onset of hepatitis. The number and size of GST-P foci increased with age and more foci were induced in males than in females. No sex difference, however, was found in the incidence of hepatitis. Although hepatitis is necessary for the induction of GST-P foci, it is insufficient for their further growth. Since hereditary hepatitis first appears at around 4 months of age, leading to a high incidence of hepatocellular carcinomas in later life, the spontaneous occurrence of the foci may be related to the development of hepatocellular carcinoma.

Using an immunofluorescent technique, we demonstrated the production of rat embryonic-type hemoglobin in extra-medullary hematopoietic cells, which appeared in the liver of adult Wistar rats fed a diet containing 0.06% 3'-methyl-4=dimethylaminoazobenzene. Specific anti-rat embryonic hemoglobin serum was prepared by immunizing a rabbit with hemoglobin from a 15- or 16-day-old Wistar rat fetus and absorbing with adult rat liver hemogenate and red blood cells. The appearance of hematopoietic foci was observed at both precancerous and cancerous stages during 3'-methyl-4-dimethylaminoazobenzene-induced liver carcinogenesis. At the cancerous stage, almost all these foci were observed in poorly differentiated hepatocellular carcinomas. The hematopoietic cells seen at both stages were stained positively with antiembryonic hemoglobin serum, but red blood cells and other types of cells in the liver did not show this specific fluorescence. These results together with earlier observations on the appearance of fetal characteristics strongly suggest that a fetal environment prevails in the adult rat liver during hepatic carcinogenesis.

Paragangliomas are tumors arising in the paraganglia and rarely occurring in the cauda equina. We report two cases of paraganglioma arising in this region. The tumors exhibited a typical "Zellballen" pattern, and immunohistochemical studies were positive for neuron-specific enolase, but negative for S-100 protein and glial fibrillary acidic protein. Electron microscopic studies confirmed electron-dense neurosecretory types of granules in the tumor cells. Only 23 cases of paragangliomas in this region have been previously reported. A brief review of the literature is included.

A remarkably high incidence of hepatocellular carcinomas was observed in long-surviving LEC rats with hereditary hepatitis. Among the 60 LEC rats examined between 12 and 28 months of age from F29 and F30, 55 (92%) developed putative preneoplastic and neoplastic lesions such as hyperplastic foci and nodules, and hepatocellular carcinomas. Of these, hepatocellular carcinomas were observed with a high frequency (46/55; 84%). All rats of advanced age that survived more than 18 months developed hepatocellular carcinomas. These results suggest that the development of liver tumors in LEC rats is an age-associated phenomenon with serial hepatic alterations after the subsidence of acute hepatitis. The long-surviving rats had no normal tissue and showed chronic hepatitis in nontumorous tissues of the liver. Cholangiofibrosis was also found in most rats with hepatic lesions. Metastasis of hepatocellular carcinomas was found in four rats. Histologically, the hepatocellular carcinomas were of a well-differentiated type with a typical trabecular structure. Thus, LEC rats seem to be a promising animal model for studying the pathogenesis of hepatitis and hepatocellular carcinoma.

Three monoclonal antibodies, PC6, PE10, and PE12, were used for immunohistochemical studies of human lungs by immunoperoxidase staining. Monoclonal antibodies PC6 and PE10 against pulmonary surfactant apoproteins stained faint granules in the cytoplasm of some alveolar wall cells in adult lung. These stained cells appeared to be alveolar Type II cells. A fetal lung of 20 weeks' gestation had no any positive staining. However, a few scattered positive cells were observed in a newborn lung of 31 weeks' gestation, and the stained cells increased progressively with increasing gestational age. The positively stained cells were very few in the lungs of newborns who died of respiratory distress syndrome (RDS), but the lungs of newborns who died of other causes after recovery from RDS showed many positively stained cells. These results suggest that the immunohistochemical demonstration of the monoclonal antibodies PC6 and PE10 could be a good pathodiagnostic indicator reflecting the localization and development of pulmonary surfactant by alveolar Type II cells. On the other hand, monoclonal antibody PE12 was found to recognize the antigen that occurs on the surfaces of the alveoli of fetal, newborn, and adult lungs as one component of the alveolar lining layer, different from pulmonary surfactant.

The induction of a novel Ca2+-dependent protease in rat liver treated with various liver promoters, as well as its increase in preneoplastic lesions during liver carcinogenesis, was demonstrated. Six groups of male Fischer 344 rats (150 g body weight) were fed separately diets containing one of the following promoters: 0.05% phenobarbital (PB), 0.05% dichlorodiphenyltrichloroethane (DDT), 0.25% ethyl-alpha-chlorophenoxyisobutyrate (CPIB), 0.5% butylated hydroxytoluene (BHT), 10 ppm 17-alpha-ethynylestradiol (EE), and 0.05% of the non-promoter diphenylhydantoin (DH). After feeding the indicated diets for 1 week, rats were killed and protease activity in the microsomal fraction of liver tissue was determined using N-benzoyl-L-tyrosine ethyl ester as substrate. The activity of protease increased 3- to 5-fold after treatment with the promoters and compared with normal liver; the non-promoter (DH) induced a slight increase in activity. Hyperplastic nodules were induced according to the method of Solt and Farber. The activity of protease was significantly high in these preneoplastic lesions compared with the surrounding liver tissue. Biochemical characterization of this protease revealed the following properties: high Ca2+ dependency, different molecular weight and optimum pH from previously reported proteases, and preferential distribution in the SER fraction. These results suggest that a novel type of protease is induced specifically in the liver by promoters of liver carcinogenesis. The possible importance of this protease in the carcinogenic process is discussed.

A serially transplantable rat hepatocellular carcinoma (HCC) line was established. The primary spontaneous HCC which developed in a 506-day-old male hereditary hepatitis LEC rat was inoculated into young LEC rats. Only this HCC of 18 primary HCCs was successful in serial transplantation. The established cell line was histologically identical to the primary HCC showing a well-differentiated type with a trabecular structure of tumorous hepatocytes. The characteristic of albumin production was maintained. Chromosome analysis revealed rather widely dispersed polyploid chromosome numbers with a modal value at 96. Every metaphase contained two to five unusually large marker chromosomes.

In contrast to its absence in normal and hyperplastic bronchial mucosa, gamma glutamyl transferase (GGT) has been demonstrated on the luminal surface of adenocarcinoma cells of bronchogenic cancer in three cases studied. On the other hand, little or no GGT was demonstrable in four epidermoid carcinomas and one oat cell cancer. Metaplastic alveolar cells appearing as cuboidal epithelial cells were uniformly positive for GGT and L-homoarginine-sensitive alkaline phosphatase. Whereas interest in GGT has centered in the past on experimental hepatocarcinogenesis, the current results demonstrate an ectopic expression of GGT in metaplastic alveolar cells and in adenocarcinoma of the lung. These findings merit further exploration to document the extent of the expression of GGT in bronchogenic cancer and to attempt to explain its presence in alveolar cell metaplasia.

The activity of butyrylcholinesterase (BCHE), a liver fetal isozyme (Zone L-V) of a nonspecific esterase, was studied histochemically and cytochemically in rat hepatocellular carcinomas induced by 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB). In normal adult rats, BCHE activity was very prominent in cells of the intestinal mucosa but was not detectable in the liver. On the other hand, in fetal rat liver, a few cells scattered throughout the organ were BCHE positive. 3'-Me-DAB induced poorly differentiated hepatocellular carcinomas showing an intense BCHE activity, especially in areas consisting of small tumoral cells proliferating in a sheet-like pattern. Surrounding noncancerous liver tissue was completely devoid of reaction products. Less-differentiated trabecular hepatocellular carcinomas also showed a positive reaction. On the other hand, well-differentiated hepatocellular carcinoma and hepatocellular carcinoma with an adenomatous pattern were barely stained, while areas of cholangiofibrosis were usually negative. Thus, in confirmation of a previous report, BCHE appears to be a positive marker of poorly differentiated hepatocellular carcinomas induced by 3'-Me-DAB. By electron microscopy, reaction products were demonstrated in the cisternae of the endoplasmic reticulum, in the nuclear envelopes, and sometimes on the cell surface of undifferentiated tumoral cells. The significance of the appearance of BCHE activity in hepatocellular carcinomas induced by 3'-Me-DAB is discussed.

To reevaluate the expression of pulmonary surfactant apoproteins in lung carcinomas, cancerous tissues from 47 cases were immunohistochemically examined by the avidin-biotin peroxidase complex method using monoclonal antibodies. These antibodies recognized apoproteins in type II pneumocytes in normal and in hyperplastic alveoli and nonciliated columnal cells (Clara cells) in terminal bronchioles of noncancerous tissues. Cancer cells positive for surfactant apoproteins were observed in 11 out of 33 adenocarcinomas, including 1 case of bronchioloalveolar carcinoma. Interestingly, two cases of adenocarcinomas with ultrastructural features of Clara cells, which had metastatic lesions, were positively stained both in the primary and the metastatic sites. Two out of 4 adenosquamous carcinomas showed positive reaction, which was exclusively localized in the cells with papillary structure. The present study indicates that the expression of surfactant apoproteins in cancer cells occurs ubiquitously in adenocarcinomas, which are considered to be derived from the terminal air way.

The activity of a novel Ca2+-dependent protease in preneoplastic and neoplastic tissues of male Fisher 344 rat liver was examined during experimental liver carcinogenesis. This protease activity is significantly high in hyperplastic nodules and hepatocellular carcinomas which were induced by the method of Solt and Farber. Taken together with our previous report on the promoter-specific induction of this protease, the results suggest the possible importance of this protease in the promotion stage of liver carcinogenesis.

We have reported an inbred strain LEC rats with high frequency of spontaneous hepatitis. In this paper, we study a possible involvement of hepatitis virus, hereditary background, and correlation between serum IgG level and onset of hepatitis in LEC rats. Neither electron microscopy nor indirect immunofluorescent test could detect hepatitis virus article or antigen. Furthermore, we could not succeed in inducing hepatitis of LEA and WKA/H rats by injecting the serum, plasma and liver homogenates of the affected IEC rats, nor in promoting/developing hepatitis of LEC and LEA rats under the immunosuppressed condition by administration of steroid hormone. When we made a mating between LEC and non-hepatitis rats to investigate the hereditary mode, F1s had no hepatitis, but F2s and backcrosses developed hepatitis. We observed, in particular, the lower the serum IgG level was, the higher the rate of developing hepatitis was. From these results, we speculate that the involvement of hepatitis virus is not likely, but that a genetic mutation might cause low level of IgG and be also correlated with development of hepatitis.

Enlarged hepatocytes with huge nuclei were found in LEC rats with hereditary hepatitis. Flow cytometric analysis of the DNA content of nuclei from jaundiced LEC rats revealed the presence of very high polyploids, such as 32n and 64n. At the age of 12 weeks, before the onset of hepatitis, 8n polyploid nuclei were more frequent in LEC rats than in LEA rats, a sibling line of LEC rats. Binucleated hepatocytes were also more frequent in LEC rats than in LEA rats at week 4. Bi-, tri- and tetra-nucleated cells whose nuclei were sometimes different in size were observed when jaundice became manifest. The number of proliferating liver cells, determined by pulse labeling with 5-bromo-2'-deoxyuridine (BrdU), was higher in LEC rats than in LEA rats at 2, 4, 8, 12 and 14 weeks, with a maximum at week 4. A remarkable increase of BrdU uptake was observed at week 16, when jaundice developed. The possible involvement of abnormal cytokinesis and kariokinesis in the manifestation of hepatitis was suggested.