An indirect inhibition assay was devised to quantitate the amount of immunoglobulin molecules on the surface of human sperm. Sperm-associated IgG was used to inhibit competitively 125I-labeled polyclonal anti-human IgG from binding to human IgG affixed to microtissue culture plate wells. Standard curves for the assay were derived by substituting soluble IgG in the supernatant of the tissue culture wells for sperm-associated IgG. The intraassay coefficient of variation was 6.7%; the interassay coefficient of variation was 5.6-21% depending upon the IgG antisperm antibody-positive plasma sample assayed. The assay technique was highly correlated (r = 0.99) with the number of antibody-bound sperm added to the supernatant. When different sperm donors were utilized as an antigen source, the coefficient of variation was 15.6% for positive plasmas and 19.3% for negative plasmas. The indirect inhibition assay showed a strong correlation (r = 0.91) with a previously described radiolabeled antiglobulin assay for sperm-associated IgG.