A recently described mouse erythroblastosis virus (MEV) was passed to newborn W/Fu rats. The rats developed generalized malignant lymphomas after 6-7 months. Two separate serial cell-free passage series were initiated. Rat lymphomas induced erythroblastosis in less than 20% of the rats that died within 4 weeks after inoculation. The surviving rats remained apparently healthy for months until they died from lymphomas. Inoculation of newborn C3Hf/Gs mice with the same Filtrates caused only lymphomas in 35-50% of the mice after 3-8 months. The lymphomas were morphologically indistinguishable from thymic lymphomas induced in mice and rats by Gross' leukemia virus. A different morphologic response was observed by serial passages of spleen filtrates or plasma from rats with erythroblastosis. All rats died within 4 weeks with erythroblastosis. In addition, multiple sarcomas and polyostotic osteolytic lesions were found in most rats of such erythroblastosis passages. The same rat erythroblastosis filtrates induced erythroblastosis and sarcomas but no osteolytic lesions in C3Hf/Gs mice. Erythroblastosis in rats was similar to the disease previously described in mice, except for a marked normoblastosis and cerebral hemorrhages which did not occur in erythroblastotic mice. The sarcomas arose in multiple sites and were histologically undifferentiated. The osteolytic lesions were likewise multicentric in origin, affecting particularly the vertebrae and long bones. Their exact nature could not be determined.