Monoclonal antibodies to human sperm were obtained from hyperimmunized BALB/c mouse spleen cells fused with myeloma NS-1 cells. Each antibody recognized definite regions in fresh unfixed sperm: equatorial region, acrosome, postacrosome, midpiece, tail. All the antibodies were specific for sperm. We selected CRL-10 monoclonal antibody, specific for acrosome, for a detailed study. The expression of the CRL-10 antibody-bound antigen was detected in other mammalian species. When CRL-10 antibody was added prior to sperm incubation in a capacitating medium, promotion of the acrosome reaction was observed.