For inbred rats with Morris hepatoma 3924A, increases in tumor size were accompanied by increases in weight and DNA content of spleen, DNA content of tibial marrow, and peripheral white cell concentrations of blood. White blood cell concentrations of rats with tumors weighing more than 5 g were approximately two-fold greater than for rats without tumors. Neutrophils were primarily responsible for the increase in white cells. Local x-radiation of 3750R to the tumor when the tumor was small prevented tumor growth and the increases in spleen weight, incorporation of 3H-thymidine into spleen DNA, white blood cell count, and tibial marrow DNA content related to tumor growth. Surgical removal of large tumors resulted in a return of spleen weight and DNA content to near normal values within 1 week. Despite the evidence for increased cell proliferation in hematopoietic tissues of rats with hepatoma 3924A, no systematic relationship has been observed between tumor size and animal survival following treatment with the cell cycle specific agent 5-fluorouracil when tumors have varied in size from 0.5 g to 5 g at the time of drug treatment.