Thymosin fraction 5 (Thymosin) has numerous immunoregulatory activities including modulation of enzymes involved in lymphocyte maturation. The effect of Thymosin on the purine metabolic enzymes adenosine deaminase (ADA), purine nucleoside phosphorylase (PNP), and 5' nucleotidase (5'NT) in null and T-enriched peripheral blood lymphocytes from sexually active asymptomatic homosexual males (AS), patients with the AIDS-related symptom complex (ARC), and those with acquired immune deficiency syndrome (AIDS) was examined and compared to its effect on lymphocytes from healthy heterosexual controls. Mean ADA activity was significantly higher in null cells from fourteen AIDS patients than in five asymptomatic homosexuals, ten ARC patients, or 27 controls. Mean PNP activity was significantly elevated in null-enriched lymphocytes from ten ARC and fourteen AIDS patients compared to controls. No differences in these enzymes were found in T-enriched cells from any group. 5'NT was markedly decreased in both null and T lymphocytes in all homosexual groups relative to controls. Homosexuals had significantly elevated percentages of OKT10 positive and Ia positive lymphocytes compared to controls. Thymosin at an optimal concentration of 150 micrograms/ml caused significant decreases in mean ADA and PNP activity in null lymphocytes from ARC + AIDS patients along with a significant decrease in the percentage of OKT10 positive lymphocytes. No phenotypic changes were seen in AS or control lymphocytes. The data suggest that Thymosin has a maturational effect in vitro on immature T cells from symptomatic homosexuals.