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Cytology and histology of recipients of allogeneic bone marrow were studied 13, 20, 24, 30 and 34 days after transplantation. The developing chronic secondary disease was characterized by increased numbers of myeloid cells, by lymphopenia and by erythroblastopenia in the bone marrow and the spleen. Erythroblastopenia together with lymphopenia and augmented myeloid cells also occurred in irradiated F1-hybrids suffering from a chronic homologous disease. The latter model eliminated the following as a cause of erythroblastopenia in secondary disease: (I) host-versus-graft reaction against donor-type erythroblasts for immunogenetical reasons, and (2) graft-versus-host reaction against recipient-type erythroblasts since they had already been destroyed by irradiation. Treatment of the donor with ALS iesulicd iii a suppression of homologous disease in F1-hybrids with a cytology resembling that of recipients of syngeneic spleen cells. The same treatment only delayed the onset of chronic secondary disease in recipients of allogeneic bone marrow. These allogeneic recipients, in contrast to the F1-hybrid recipients, died with the typical morphology of a chronic secondary disease. In recipients of syngeneic bone marrow from donors treated with ALS, repopulation with lymphocytes was somewhat delayed and transient erythroblastosis in the spleen occurred 20 days after transplantation. [ABSTRACT FROM AUTHOR]