Using the polyethylene glycol (PEG) precipitation technique, circulating immune complexes were demonstrated in 88% of patients with acquired immune deficiency syndrome (AIDS) and AIDS-related disorders. The finding of immune complexes in these individuals was confirmed by using a second assay, the commercially available Clq-IgG enzyme-linked immunosorbent assay kit. The material precipitated by PEG contained elevated levels of IgG, IgM, Clq, and C3. There was no substantial correlation between the level of IgG in the complexes and the serum IgG concentration, although there was a significant association between complexed IgG and the titer of anti-AIDS-associated retrovirus (ARV) antibody in the serum. In some precipitates, ARV antigens were detected by immunoblot analysis. Coculture of other precipitates with normal lymphocytes revealed the presence of infectious virus. These data confirm the existence of immune complexes in the sera of AIDS patients and indicate that in some cases they may comprise ARV and anti-ARV antibodies.