The insoluble granular material of the acrosome vesicle of sea urchin sperm has been isolated and shown to be a single 30,500 dalton protein for which the name "bindin" is proposed. The data presented are consistent with the hypothesis that bindin is the adhesive responsible for the attachment of sperm to the vitelline layer of the egg. Experimental results suggest that bindin may act by binding to carbohydrate receptors of vitelline layer glycoproteins. The speculation is made that sperm bindins may be the general mechanism by which animal sperm attach to eggs.