Inhibin isolated from human seminal plasma which has 94 amino acids has been shown to be structurally similar to a sperm coating antigen of prostatic origin. Specific antibodies generated against this peptide caused agglutination of human sperm. Using FITC-labeled antibody, antigen was localized on the post-acrosomal head region of sperm. Antiserum to inhibin could also impair the penetration of human spermatozoa into cervical mucus. After 10 and 30 minutes, the depth and density of penetration as well as the motility of the sperm were inhibited. The treatment of sperm with antiserum to inhibin caused an inhibition of sperm attachment to the egg as well as inhibition of penetration.