A glycoprotein antigen of 80,000 apparent molecular weight expressed by S. mansoni eggs, cercariae, and male and female worms has been characterized by use of a monoclonal antibody prepared against cercarial glycoproteins. The antigen was expressed on the surface of both male and female worms. Its expression on the male surface, however, was restricted to the gynecophoral canal of male worms derived from fully patent infections. This pattern of surface expression suggests a role in schistosome sexual reproduction.