One hundred and sixty seven combinations of viral+viral antibodies or viral+bacterial antibodies were tested for their ability to precipitate each other. Some antibodies produced against HIV epitopes recognize and precipitate some antibodies produced against cytomegalovirus (CMV), hepatitis B virus (HBV) core antigen, and Mycobacteria tuberculosis (MTb) and Staphylococcus epitopes but not those against HBV surface antigen, herpes simplex types 1 and 2 (HSV1 and HSV2) or Epstein–Barr virus (EBV), Streptococcus, or Escherichia coli. In addition, CMV antibodies precipitate those of HBV core and surface antigens as well as MTb, but not HSV1, HSV2, EBV, Streptococcus or E. coli. HBV core (but not surface) antibodies precipitated Mycobacterium avium antibodies (MAv) but not MTb, Streptococcus, Staphylococcus or E. coli antibodies. Binding constants vary between k^ds of 10^-9 and 10^-7 M. Interactive antibodies act like idiotype–antiidiotype pairs suggesting that the inducing antigens are molecularly complementary. The resulting antibody interactions may explain the formation of circulating immune complexes that are commonly found in AIDS and in other diseases characterized by multiple, concurrent infections. This observation suggests that AIDS pathogenesis may involve autoimmune mechanisms in which the immune system attacks itself to form antibody–antibody circulating immune complexes that contribute to the hypergammaglobulinemia characteristic of AIDS. Complementary cofactor infections in AIDS may therefore contribute to the immunosuppression of the syndrome and difficulties treating these corresponding infections.