Five hundred and fifty six subjects, known to be homosexuals or intravenous drug abusers and seropositive for HIV antibody, were selected on the basis of their clinical state--symptom free, lymphadenopathy syndrome (LAS), AIDS related complex (ARC), and AIDS. The presence of antigenaemia and the humoral response to viral polypeptides was investigated. The prevalence of patients positive for p31 antibody was significantly increased in those with AIDS and detectable antigenaemia.