A schistosomula surface antigen of 38,000 daltons has been isolated by using a rat monoclonal IgG2a that has been shown to confer protection against S. mansoni infection by passive transfer in rats. This antigen is one of the previously characterized surface proteins reacting with sera from various infected hosts, including rat, mouse, monkey, and human. Studies on parasites of different developmental stages indicated the presence of this antigen on cercariae and skin-derived or mechanically transformed schistosomula.