The present investigation was carried out to study the effect of tumor growth on the spleen of an aged host. Dalton's lymphoma (DL), a spontaneous T cell lymphoma, was grown in mice of different age groups classified as young, adult or old on the basis of their reproductive status. Splenocytes obtained from normal and tumor-bearing young, adult and old mice were checked for an in vitro blastogenic response to concanavalin A (Con A), colony-forming ability and apoptosis. There was an enhanced apoptosis of splenocytes and a concomitant inhibition of splenocyte blastogenesis and their responsiveness to the mitogenic stimulus of Con A in aged mice. The counts of granulocyte macrophage- and macrophage-colony forming units were significantly enhanced in the spleen of tumor-bearing adult mice. It is proposed that the DL-growth-dependent increase in the size of the spleen in adult mice is due to an increased blastogenesis of splenocytes, which, however, may not be applicable in the case of old tumor-bearing mice. The role of splenic macrophages in the regulation of the functions of the spleen by macrophage-derived NO is shown.