Passive immunization of female mice by the intraperitoneal administration of ascitic fluids containing monoclonal antisperm antibodies blocked the development of mouse zygotes to morulae and blastocysts when cultured in vitro. Radioiodinated YWK-I monoclonal antibodies were prepared and administered intraperitoneally to female mice. Significant radioactivity was found in washes of the oviductal lumens. The results suggest that antisperm antibodies block fertilization and may have a deleterious effect on early developing embryos.