The incidence of circulating immune complexes (CICs) was evaluated in sera from 39 female partners of infertile marriages and from 38 fertile women. Fifteen (38%) of the infertile women had CICs, as determined by the Raji cell assay, in levels ranging from 300 to 8000 micrograms/ml; whereas only 1 (3%) of the fertile women displayed CICs (P less than 0.001). Analysis of the CICs from nine of the women following polyethylene glycol precipitation and acid dissociation revealed that four contained C1q and three contained an antigen reactive with rabbit antibody to human spermatozoa. These latter three women all lacked free sperm antibody, as determined by enzyme-linked immunosorbent assay and agglutination. Thus, CICs are not uncommon as a manifestation of infertility in females. Their presence may lead to an underestimation of sperm antibody levels and may be indicative of underlying infection or autoimmunity.