M.1 monoclonal antibody has previously been shown to passively transfer partial resistance to schistosome infection within mice and to recognize a 28-kDa antigen that has peptide sequence homology with triose-phosphate isomerase (TPI; D-glyceraldehyde-3-phosphate ketol-isomerase, EC 5.3.1.1). We have now isolated the complete coding DNA for Schistosoma mansoni TPI and confirmed that this cDNA encodes the 28-kDa antigen recognized by M.1. The predicted translation product has strong homology with other TPIs, particularly from higher eukaryotes, and the sequence homology is greatest in regions known to form the active site. The complete coding DNA has been expressed within an Escherichia coli host to produce high levels of soluble, recombinant S. mansoni TPI protein. The product is recognized and purified by the M.1 antibody and is a functional TPI with an intrinsic specific activity comparable to that of rabbit and yeast TPI.Schistosomiasis is a chronic, often debilitating, parasitic disease afflicting several hundred million individuals worldwide. In addition, related schistosome parasites that infect livestock cause a serious hardship in many third-world nations. The disease is characterized by the presence of adult worms, or blood flukes, within the portal and mesenteric veins or within the veins of the bladder. The primary pathological consequences of schistosomal infection are caused by the host's granulomatous immune response to the eggs, produced by the male/female worm pairs.Despite having effective immune avoidance mechanisms, it is the consensus view that schistosomes induce, upon infection of human and rodent hosts, variable levels of resistance to reinfection (1). The partial immunity can be passively transferred, in rodents, by immune sera and by monoclonal antibodies against a variety of different schistosome antigens (see refs. 2 and 3 for reviews). In several cases, the antigens identified by the partially protective monoclonal antibodies have been used as immunogens to induce partial protection to infection. It is hoped that one or several ofthese antigens, produced through recombinant DNA technology, might form the basis of a schistosomiasis vaccine. A vaccine inducing even partial protection could have a major impact in endemic areas by decreasing the disease pathology and slowing transmission rates.Monoclonal antibody M.1 recognizes a 28-kDa antigen found in all life cycle stages of schistosomes (4). The antibody imparts a significant, though partial, level of protection against parasite challenge after passive transfer into naive mice (4). Immunization with immune complexes containing the 28-kDa antigen bound to Ml were found to induce a 38% reduction (P < 0.05) in parasites after challenge infection (5). Recently, the M.1 antibody was used as an immunoaffinity reagent to purify the 28-kDa antigen (6). Several tryptic peptides of the purified protein were subjected to aminoterminal sequence analysis and their sequence revealed strong homology to mammalian and microbial triosephosphate...