Forty-six monoclonal antibodies were produced against the preacetabular gland secretions of Schistosoma mansoni cercariae by two different immunization protocols. These antibodies were of both the IgG and the IgM classes. One IgM monoclonal antibody (Ia4D6) was further characterized. It was specific to the cercarial stage by ELISA and showed specific binding to the 30,000 Mr proteinase in crude cercarial secretions by Western blot analysis. Preincubation of this antibody with purified cercarial proteinase resulted in inhibition of proteolytic activity, and it mediated complement-dependent cytotoxicity to cercariae in vitro. Immunoperoxidase staining of cercariae localized this antibody to vesicles visible within the preacetabular glands and their secretory ducts, and to secreted material. ELISA and Western blot analysis also showed that sera from infected mice and patients with schistosomiasis reacted with the cercarial proteinase. These studies demonstrate that a proteinase secreted into the host by invading cercariae is immunogenic and provide a monoclonal antibody probe for further characterization of its structure and function.