A single intradermal injection of frozen and thawed schistosomula in conjunction with the bacterial adjuvant Mycobacterium bovis strain Bacille Calmette Guerin, Phipps substrain (BCG) induced significant levels of resistance to challenge Schistosoma mansoni infection in C57BL/6 mice. Immunization with the aqueous fraction remaining after 100,000 X G centrifugation of the larval lysate was also protective under these conditions, suggesting that some immunogenic determinants may not be membrane associated. Frozen-thawed cercariae and soluble components of adult worms also protected against challenge infection in these experiments. These observations indicate that soluble immunogens are present in both early and late developmental stages of the parasite, and therefore may be good candidate antigens for an immunochemically defined vaccine against schistosomiasis. Induction of humoral reactivity against soluble or membrane antigens was examined in mice protected against cercarial challenge by prior exposure to frozen-thawed larvae, soluble larval, or soluble adult antigens plus BCG. Animals that were immunized with frozen-thawed larvae produced low but significant levels of antibodies against larval surface antigens when examined by indirect immunofluorescence or by immunoprecipitation of surface-labeled schistosomula. Mice immunized with soluble antigens, however, showed negligible antibody reactivity against surface membrane antigens. Because mice immunized with soluble antigens were resistant to challenge infection, these results strongly suggest that anti-surface membrane reactivity is not required in the mechanism of protective immunity in this model. Sera from mice immunized with either total freeze-thaw larval lysate or soluble schistosome extracts all showed strong reactivity against soluble antigens, as detected by ELISA. Western blot analysis showed these antisera to react with a restricted number of high m.w. antigens that were present both in schistosomula and in adult worms. These antigens are therefore likely to play a major role in the development of resistance in this model as immunogens and/or as targets of protective immune response.