We studied serological aspects of autoimmunity in patients with AIDS, AIDS-related complex (ARC) and in individuals at risk for AIDS. Immunoglobulin (Ig) M, IgG and IgA rheumatoid factors (RF) were quantified by enzyme-linked immunosorbent assay (ELISA), Ig by radial immunodiffusion, and circulating immune complexes (CIC) by the CIC-conglutinin and CIC-complement 1q (C1q) assays. Mean IgM RF levels were normal in AIDS patients, but those of ARC patients were higher and more frequent than the levels defined by agglutination methods. Similar observations were made for intravenous drug users (IVDU) and for both HIV-seropositive and HIV-seronegative homosexual men. Mean IgG RF levels were normal in AIDS and ARC patients but high in homosexual men and, to a lesser degree, in IVDU. IgA RF levels were high in many AIDS and ARC patients, in homosexual men, and in haemophiliac and control groups. The selective increase of the IgA isotype in AIDS was confirmed by the Ig results, which also showed an IgG increase in all groups. IgM were mainly high in people with ARC. CIC were detected in 68% of ARC patients by both methods, and in 55% of AIDS patients by CIC-CIq. A high incidence of positive samples in all at-risk populations, but particularly in seronegative individuals, was observed using CIC-conglutinin. CIC-C1q also revealed larger amounts of CIC in HIV-seronegative individuals, mainly in homosexual men. The study of these humoral aspects of autoimmunity provides useful information on the impairment of B-cells in patients with AIDS and ARC. Our findings concerning groups at risk suggest a relationship between production of CIC and IgA RF and seroconversion in homosexual men.