AbstractMouse zonae pellucidae contain receptors for sperm throughout their structure since spermatozoa will bind to both the inner and outer surfaces of isolated zona fragments. Antibodies raised against mechanically isolated mouse zonae pellucidae were only capable of suppressing sperm binding to the outer zona surface in association with the formation of a precipitate in this region.These results indicate that such antisera are not capable of interacting directly with the sperm receptors on the zona pellucida but rely upon the less efficient mechanism of steric hindrance to prevent sperm from gaining access to the sperm binding sites.