Two immunologically cross-reactive plasma membrane proteins, of Mr 80,000 and Mr 210,000, have been purified to apparent homogeneity from sperm of the sea urchin Strongylocentrotus purpuratus. Purification includes a combination of antibody and wheat germ agglutinin affinity chromatography. The two proteins have similar but not identical amino acid compositions; however, their carbohydrate composition differs substantially. After purification, the Mr 210,000 protein binds to both living eggs and isolated egg jelly in a species-specific manner, but the Mr 80,000 protein does not. The inactivity of the Mr 80,000 protein could be due to denaturation during purification. The data are consistent with a model in which the Mr 210,000 protein acts as a jelly receptor in the sperm membrane, promoting the ion movements necessary to initiate the sperm acrosome reaction.