We have previously demonstrated that a Fasciola hepatica-derived adult worm antigen, which is cross-reactive with Schistosoma mansoni and designated FhSmIII(M), induces resistance to challenge infection with S. mansoni in mice. The current review concerns the methods developed to isolate and partially characterize a major component of FhSmIII(M), a 12-kDa polypeptide, as well as immunity studies involving this antigen. Utilizing conventional gel filtration, followed by diethylaminoethyl (DEAE) Sephadex A-120 and monitoring the fractions by polyacrylamide gel electrophoresis (PAGE) and enzyme-linked immunoelectrotransfer blot techniques (EITB), we were able to isolate the 12-kDa antigenic polypeptide to homogeneity. Conventional gel filtration chromatography was followed by high-pressure, liquid anion, exchange chromatography, when highly purified material was needed, although the effective yields diminished drastically with the latter. Mice, rabbits and calves with a primary infection of F. hepatica developed antibodies (detectable in enzyme linked immunosorbent assay (ELISA) to the F. hepatica 12-kDa polypeptide within 2 weeks of infection. Mice with a primary infection of S. mansoni developed significant, but low, levels of anti-12-kDa antibodies by 7 weeks post-infection. Immunization of mice with microgram amounts of this 12-kDa polypeptide in Freunds' adjuvant resulted in the development of up to 77% less S. mansoni worms than the controls. Treatment with either endoglycosidase H, neuraminidase or dithiothreitol had no effect on the protein's mobility on sodium dodecyl sulfate (SDS)-PAGE or in its recognition by antibodies, suggesting the absence of carbohydrate moieties or disulphide bonds in relation to its antigenic determinants. Degradation by proteinase K further confirmed its polypeptide nature and points to recombinant DNA technology for the large-scale manufacture of this potential vaccine. Further use of this antigen in immunity studies should greatly contribute to the clarification of the mechanisms involved in cross-resistance against schistosomiasis.