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1. Macaca mulatta monkeys on a normal diet have proved resistant to intranasal but not to intratracheal inoculation of influenza virus. 2. Neutralizing antibodies appeared 8 to 10 days after inoculation with either living or heat-inactivated virus. The antibodies were noted to be still present as long as 9 months after infection with living virus. 3. A specific granulopenic leucopenia characteristically followed primary influenza virus inoculation, regardless of altered conditions of diet, exposure, and route of inoculation, but it was not observed in monkeys previously infected with the same virus, all of which invariably survived. 4. Nutritional deficiency and exposure to cold increased the susceptibility of monkeys on intranasal instillation of the virus; the leucopenia was profound and fatalities frequently occurred even though neutralizing humoral antibodies developed as promptly and in relatively the same titer as under optimum nutritional conditions.

1. The vital importance of the cellular defense forces in the resistance of the monkey to combined streptococcus and influenza virus infections has been demonstrated. 2. Some of the conditions prejudicial to the maintenance of an optimum cellular reserve in the infected animal have been revealed; viz., undernutrition, physical cold, intratracheal route of infection. 3. The potential threat exerted by latent foci of streptococci, and the importance, in relation to the combined infection with virus, of cellular and humoral immunity, together or separately, have been demonstrated. The essential rôle of optimum nutrition (notably as concerns the vitamin B complex, and folic acid specifically) in the prevention of disastrous illness from these infectious agents, individually or in combination, would seem to have been proven. 4. Signs of glomerular nephritis appeared in a significant number of monkeys receiving Streptococcus hemolyticus and influenza virus in sequence, followed by reinoculation or spontaneous reappearance of the streptococci. 5. Reinoculation of Streptococcus hemolyticus, group C, resulted in a prompt "booster" increase in the opsonic index. Virus instillation was followed by just as sudden a depression in this index. 6. Reinoculation failed to evoke either the granulocytosis or the leucopenia in monkeys which are characteristic effects of the streptococcus and the virus respectively when these agents are introduced for the first time by way of the nasal mucous membrane. 7. Simultaneous intranasal inoculation of influenza virus, type A, and Streptococcus hemolyticus, group C, in nutritionally normal Macaca mulatta failed to produce obvious signs of disease. In most of the animals, however, a streptococcus-induced leucocytosis followed by a delayed virus-induced granulopenia developed. 8. Inoculation of influenza virus followed in 4 to 17 days by streptococci produced obvious signs of disease in five of eleven animals which had become leucopenic as result of the action of the virus, and fatal streptococcal septicemia in two monkeys. 9. The development of signs of infection in previously healthy monkeys exposed to virus followed by streptococci confirms both the clinical and laboratory experience of other observers, that virus infection may predispose to secondary bacterial invasion, and, that at times, under unfavorable circumstances, the infection may become overwhelming. Although the complete mechanism of resistance is as yet not wholly clear, the depressant or inhibitory effect of the virus on both its cellular and humoral elements has been established.

Macaca mulatta on a normal diet proved to be resistant on intranasal inoculation with Streptococcus hemolyticus, group C. More than half of the monkeys maintained for various periods on a modified dietary regimen deficient in vitamin B became more or less anemic. All developed a granulopenic leucopenia during which many developed spontaneous infections, and all were extremely susceptible on intranasal instillation of Streptococcus hemolyticus, group C. In normal monkeys, primary inoculation with the streptococcus resulted in an immediate marked neutrophilic leucocytosis. In the nutritionally deficient animals with leucopenia, only an abortive transitory leucocytosis was observed. Following reinoculation with the same strain of hemolytic streptococci in normal healthy monkeys, the prompt leucocytic response originally observed, failed to develop, yet the animals remained asymptomatic. In the first instance the opsonic index remained low at the preinoculation base line level, but in the second, a sharp increase in the opsonic index occurred in several days. This humoral response apparently rendered unnecessary the quantitative granulocytic leucocytosis associated with survival in the first instance. There was no demonstrable difference in time of appearance or titer of the antibody responses, i.e. precipitins, antistreptolysins, and opsonins between the normal and nutritionally deficient monkeys. Synthetic folic acid (L. casei factor) relieved both leucopenia and anemia promptly in the "vitamin M"-deficient monkeys, with increase in resistance to endogenous infection when this supplement was given sufficiently promptly.

The PR8 strain of human influenza virus was found to proliferate and disseminate widely in the tissues of fetal guinea pigs inoculated in utero. Large quantities of virus free of bacteria were recovered from lung, liver, and placenta, and smaller quantities from blood and brain, after incubation periods ranging from 2 to 6 days. Although the fetuses proved to constitute an excellent medium for the propagation of influenza virus, they evinced little gross reaction to the infection. Several series of passages from fetus to fetus were accomplished; one consisted of 10 transfers, another of 16. For serial passage the virus was inoculated intracerebrally into half-grown fetuses and the fetal lungs were harvested 48 hours later as a source of virus for subinoculation. It is concluded that multiplication of the virus occurred particularly in the lungs, which may be considered a significant reaffirmation of the pneumotropic tendencies of this virus. Following passage in series the virus was found, on the basis of cross-immunity and cross-neutralization tests, to be immunologically identical with the mouse passage virus from which it was derived. Other properties also appeared to be unaltered by passage of the virus under these conditions.

Experiments were carried out to determine the relative susceptibility of guinea pigs at different ages to the virus of epidemic influenza. From a correlation of these studies on the mature fetus, the newborn, and the adult animal, with previously reported findings on the immature fetus, we draw two conclusions: first, that there is a gradually increasing resistance to infection with this virus during intrauterine development, with but little change thereafter; and second, that at the time of birth there is a sudden loss of infectibility by routes other than the intranasal. These results illustrate then the benefits which may accrue if one projects into the period of antenatal life studies dealing with the age factor in relation to susceptibility to infection. It is implied that data collected from observations of the postnatal animal alone are of necessity incomplete and may be misleading.

1. A method is described for propagation of bacteria-free vaccinia virus in the rabbit fetus. 2. For the production of severe generalized infection, affording a high yield of virus, the following conditions were found to be favorable: the use of 24 day fetuses, a virus dosage of 10,000 adult rabbit skin units, injection into multiple sites, and an incubation period of 4 days in utero. 3. Under these conditions the virus was found to disseminate widely in the infected fetus, being recoverable particularly from liver, lungs, brain, skin, placenta, and kidney. 4. Under varied conditions of virus dosage, fetal age, and incubation period, all grades of reaction were observed in the fetus, ranging from the mildest infection, with only an occasional small lesion demonstrable, to generalized vaccinia with pocks in nearly every organ of the body, and often death. 5. It was shown that the virus could be carried in serial passage through fetuses. A series of 27 such passages was made by using fetal skin for subinoculation at each transfer. Similarly, two other strains of the virus were evolved in shorter series of transfers by using respectively fetal brain and fetal liver as the subinocula. 6. The passage strains maintained their identity and titer, as judged by the intradermal inoculation of adult rabbits. However, these strains underwent a significant reduction in their capacity to infect by the scarification method, and the lesions induced by intradermal inoculation were much milder than those produced by the parent strain. None of the passage strains manifested properties characteristic of the so called neurovaccine strain, despite the frequent use of intracerebral inoculation in the fetuses. 7. In explanation of the reduction of virulence of the passage strains, it is suggested that a given suspension of virus may be considered as consisting of particles of varying degrees of virulence, and that when cultivation is carried out in highly susceptible tissues a relative overgrowth of the less virulent elements is permitted, so that apparent attenuation results.