A monoclonal antibody has been used to identify and characterize an antigenic tegumental surface membrane glycoprotein of Schistosoma mansoni. Direct binding of 125I-labeled monoclonal antibody showed that this glycoprotein was present in eggs, cercariae, and worms of both sexes. The glycoprotein had an apparent molecular weight of 180,000. Indirect and direct immunofluorescent microscopy showed that this antigen was located on the interlinked tegumental folds of both larval and adult parasites. These findings are discussed in relation to parasite development and the mechanism by which schistosomes evade the host's immune defenses.