Immunological tests of cell-mediated immunity (CMI), serological tests for antibodies to sexually transmitted and possibly immunosuppressive viruses, determination of serum immunoglobulin levels, full blood counts and serotyping for the HLA-DRw5 antigen were performed in 10 homosexual men with a mean age of 35 years (range 21-59 years). Five of these were associates of a patient who had died of confirmed acquired immunodeficiency syndrome (AIDS). At the time of investigation 9 of the men were apparently healthy and 1 had active pulmonary tuberculosis. Only in 2 subjects were no abnormalities of CMI found; they were 23 and 28 years of age and, interestingly, were also the only 2 whose sera were negative for antibodies to cytomegalovirus (CMV) and for the other viruses investigated. Of the remainder, 1 was lymphopenic, 6 had increased numbers of circulating suppressor T lymphocytes, 4 had reduced delayed-type hypersensitivity skin responses to recall antigens, and in 6 mitogen-induced lymphocyte transformation was decreased. Of these 8 subjects, 4 and 2 had antibodies to hepatitis B surface and core antigens respectively and 1 was positive for hepatitis B surface antigen and HBe antigen. The most severe abnormalities were observed in individuals who were seropositive for both CMV and hepatitis B virus. These findings indicate the existence of acquired immunosuppression in some members of the South African homosexual community. On the basis of these findings an immunological classification of potential sufferers of AIDS in the male homosexual community is proposed.