Mice protected against Schistosoma mansoni infection by intradermal (i.d.) vaccination with nonliving schistosomula or soluble extracts of larval or adult schistosomes (SCHLAP and SWAP, respectively) produce antibodies that react by Western blot analysis with one antigen of Mr (X 10(-3)) 97 in SWAP prepared in the presence of protease inhibitors and two antigens of Mr (X 10(-3)) 95 and 78 in SWAP prepared in their absence. Vaccine antibodies also immunoprecipitated a single 97k molecule, with a pI of 5.5, from detergent extracts of [35S] methionine-labeled schistosomes. Three hybridomas, produced from spleen cells of i.d. immunized mice, all recognized both the 95k/78k doublet and the 97k antigen, indicating that the two lower Mr components are degradation products of the same 97k molecule. The 97k/95k/78k complex (Sm-97) was purified by affinity chromatography and found to constitute 0.5% of the total protein in SWAP. 125I-concanavalin A bound weakly to purified Sm-97, indicating that this antigen is minimally glycosylated. By indirect immunofluorescence, Sm-97 was localized to regions just below the tegumental and gut syncitia of adult worms. Mice protected by i.d. vaccination produced high titers (1:10,240) of anti-Sm-97 antibodies, whereas chronically infected mice responded at a much lower level (titer 1:640). In contrast, mice protectively immunized with irradiated cercariae and mice nonprophylactically inoculated by the i.v. route failed to produce detectable anti-Sm-97 antibodies. Competitive radioimmunoassays performed with 125I-labeled monoclonal antibodies and purified antigen defined at least two distinct epitopes on Sm-97. Antibodies from i.d. vaccinated mice recognized both monoclonal antibody-defined epitopes, whereas anti-Sm-97 antibodies in chronic infection sera recognized neither. Finally, purified Sm-97 was shown to elicit delayed-type hypersensitivity in i.d. vaccinated mice, suggesting that this molecule is also capable of evoking cell-mediated responses, a finding consistent with its proposed function as a vaccine immunogen.