Twenty-two HIV-positive homosexual men with a moderately impaired immune system were randomized to bestatin capsules 60 mg a day or placebo for 4 weeks. None suffered from opportunistic infections. The immunomodulating effect of bestatin was investigated by lymphocyte proliferation assay with pokeweed mitogen, phytohaemagglutinin, and concanavalin A, surface marker analysis of T-helper/T-suppressor cells by flow cytometry, natural killer cell activity, beta-2-microglobulin and HIV antibody quantitation. No significant differences between bestatin and placebo in immunological, haematological, biochemical or clinical variables were detected.