We have evaluated the immunomodulatory effects of isoprinosine in a double blind randomized clinical study on 63 immunosuppressed male homosexuals with persistent generalized lymphadenopathy (PGL) or ARC. The subjects received either placebo or isoprinosine at 1 or 3 g/day for 28 days. All subjects were monitored for performance for a one year period. In the 3 g/day treatment group clinical improvement was reported by 52% of the patients in contrast to 15% in the placebo group. Patients receiving 3 g/day isoprinosine showed significant increase in NK cells, a major subset of which bears the Leu 7 surface antigen, and in NK cell function as early as at the termination of treatment. This normalized NK cell property was still evident 5 months after cessation of therapy. Total T lymphocytes and T helper cells also increased in this group and a concomitant reduction was observed in activated T lymphocytes (HLA-DR+). As a direct result of the therapy an increase was found in the Th regulatory (Leu 3+ Leu 8+) cell population resulting in normalization of Th inducer/Th regulatory cell ratio. A concomitant reduction to normal range occurred in Ts effector (Leu 2+ Leu 8-) and functionally activated Ts (Leu 2+ HLA-DR+) cell populations. The kinetics of these effects suggest that isoprinosine stimulates the production of precursor lymphocytes and initiates a process of cell differentiation capable of producing long-term restoration of host immunity.