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A "sandwich"-type radiolabeled antiglobulin assay using monoclonal anti-C5b-9 neoantigen and polyclonal anti-C5b-9 was used to evaluate the presence of terminal C complexes (SC5b-9 or MC5b-9) in the sera and ovarian follicular fluid (FF) from 45 infertile women. FF SC5b-9 was detectable in all clinical diagnostic categories. The mean SC5b-9 levels in FF and sera were 399 ng/ml (range 75 to 1350 ng/ml) and 798 ng/ml (range 0 to 2700 ng/ml), respectively. Twelve (26.6%) of the 45 FF samples had normal hemolytic C activity, and all FF (n = 44) samples initiated C8/C9-dependent lysis of sensitized sheep E coated with human C1-7. Human plasma IgG antisperm antibodies (ASA) were capable of activating C in 31 (72%) of 43 FF samples as detected by their ability to deposit MC5b-9 on human sperm. Sera from infertile women with ASA in their sera and FF impaired human sperm binding to human zona pellucida and binding and penetration of zona-free hamster oocytes in vitro. The discovery of SC5b-9 and MC5b-9 in ovarian FF implies that the interaction of ASA and C could have a deleterious effect on sperm during in vivo and in vitro sperm-egg interactions in women with antisperm antibodies.