The distribution of T cell subsets in pubertal (2 months) and post-pubertal (10 months) mice showed a significant decrease in the percentage of CD4+ splenocytes and peripheral blood lymphocytes (PBL) with age, unlike the percentage of CD8+ cells in PBL, which remained unchanged. The change in the distribution of T cell subsets in the spleen and blood occurred in 2 months old castrated mice, as in 10 months old animals. P388 tumor grew better in post-pubertal and in castrated mice than in young mice. The intact mice survived longer than the castrated ones. The relative number of CD4+, CD8+ and CD2+ splenocytes was lower in transplanted intact mice than that in controls. The CD8+ and CD2+ subsets in the blood of 2 months transplanted mice were higher than those in controls, whereas in PBL, in 10 months old and castrated mice, the T lymphocyte subsets remain unchanged. Depo-testosterone (DT) injection strongly reduced weight and tumor growth in all the intact and castrated animals. A significant correlation is observed between the tumor weight and testosterone level in the plasma of the 2 months old DT treated mice. Moreover, DT injection induced a significant increase in the percentage of blood CD8+ cells in all the batches. These data indicate that physiologically, androgens affect the age-related distribution of lymphocyte T subsets and suggest that they slow down tumor growth, besides causing a direct effect, through an immunological process.