While studying the plasma membrane of sperm of the sea urchin Strongylocentrotus purpuratus, we developed an antiserum that exhibits unusual reactivity--it cross-reacts with the surfaces of spermatozoa of 28 species representing seven phyla of the animal kingdom. A negative cross-reaction has not been found. This suggests the possible existence of common antigenic determinants on the surface of all animal sperm. We report these preliminary results here because of the broad implications common sperm-surface antigenicity has for the potential development of immunocontraceptive methods.