Peripheral blood leukocytes (PBL) were drawn from a number of age matched heterosexual and homosexual men from the Washington, DC area, and New Mexico. The PBL were sensitized in vitro to influenza virus and to HLA alloantigens. These sensitized cultures were tested for the generation of cytotoxic T lymphocytes (CTL) specific for influenza virus and alloantigens. Assays were also run for OKT4:OKT8 ratios (i.e., helper:suppressor cell), and for serum antibodies to HTLV-III as well as for the presence of HTLV-III virus. Similar studies were performed on AIDS patients and on donors with AIDS-related complex (ARC). Approximately 60% of homosexual men from both cities exhibited elevated CTL activity to HLA alloantigens. Among the AIDS and ARC patients, all exhibited a lack of CTL activity to influenza, although approximately 50% of these donors exhibited elevated CTL activity to alloantigens. In vitro studies demonstrated that leukocytes from unifected donors could be more efficiently infected with HTLV-III when the cultures were stimulated with HLA alloantigens than with T cell mitogens. This was particularly true in donors who exhibited elevated allogeneic CTL activity.