Blood cell counts, differential blood cell counts and weights of the spleen and peripheral lymph nodes draining the area of lesions induced byMoloney sarcoma virus inoculation into the quadriceps shank muscles of inbred BALB/c mice were examined at various stages of tumor development and regression. The blood cell count remained constant through the observation period up to 27 days post tumor development and regression. Differential counts revealed some changes in the cellular composition of the peripheral blood. The most pronounced was an increase of neutrophiles at the stage of tumor development, and their decline with tumour regression. The enlargement of the spleen and of the popliteal lymph nodes draining the tumour site, at the peak of tumor development on day 13 post MSV inoculation, involved at least a doubling of splenic weight, and a much greater weight increase for lymph nodes. This was a long-lasting, although declining event, extending beyond tumor regression.