The effects of testosterone on the relative proportion of Thy 1.2, CD4 (L3T4) and CD8 (Lyt-2) cells in thymus and spleen were studied after castration and administration of Depo-testosterone (DT) separately or together with cyproterone acetate (CA) (an antiandrogen) in BDF1 mice. Injection of 0.5 mg/100 g body weight of DT during 2 weeks decreased significantly the number and proportion of double positive (DP) (CD4+ CD8+) and increased the percentage of single positive (SP) CD4+ (CD4+ CD8-), whereas there was a slight decrease in the Thy 1.2+ cells in the thymus. In parallel, we observed an increase in CD8+ (CD4- CD8+) cells in the spleen. The androgen deprivation after 3 weeks of castration induced a decrease in the percentage of CD4+ cells in thymus and both CD4+ and CD8+ cells in spleen. Injection of CA (0.5 mg/100 g body weight) had the same qualitative effects as DT on the proportion of lymphocyte T subsets in castrated mice. However, the combined activities of DT and CA were greater than either alone. These data indicate the main role of testosterone in the distribution of CD4+ and CD8+ cells in male mice. The similar effects of CA and DT in the lymphoid organs may suggest a difference between androgen receptors of sexual and lymphoid organs.