An antigenic spermatozoal polypeptide was identified from nonionic detergent solubilized membrane material of sperm cells by using a human seminal plasma specific rabbit antiserum. The radiolabeled sperm cell membrane preparation was subjected to gel filtration and gel fractions were assayed for the binding activity with antiseminal plasma antiserum. The immune complex of the peak fraction of the binding was applied on SDS-PAGE, and the antigen was estimated to be a single polypeptide of approximately 12,000 daltons. The inhibition assay of the binding revealed that the antigen is present in sperm, seminal plasma, and milk, but is absent from any other organs and tissues except male genital tracts. Seminal plasma showed the largest amount of the activity, suggesting that this antigen is a sperm coating antigen. In seminal plasma, the antigen molecule present in a large form with a molecular weight of approximately 50,000 daltons. The antigenicity was stable when treated with heat, acid, alkali and various protein denaturants. The characteristics of the present antigen are very similar to those of lactoferrin and ferrisplan, the other sperm coating antigens common to milk. However, they are quite different in molecular weight, and no cross reactivity was found between the present antigen and lactoferrin or ferrisplan.