SummaryThe tegumental membranes of adultSchistosoma mansonihave been isolated and purified and shown to function as potent immunogens; they elicit an essentially identical immune response in rabbits, rats and mice. Anti‐membrane antisera harvested from these animals consistently recognized common antigens, of relative molecular weight (mol. wt) 32000 and 20000, on the surface of young schistosomula, 5 day old lung worms and adult worm purified membranes. An additional molecule of 25 000 mol. wt was present on the surface of lung worms and adult worm membranes and was specifically recognised by serum from chronically infected mice and by serum from rabbits inoculated with adult worm purified membranes. The concept of antigenic identity between developmental stages that parasitize the mammalian host was further substantiated by the observation that anti‐membrane antiserum bound to live schistosomula, lung worms and adult parasites as measured by indirect immunofluorescence. In complement‐mediatedin vitrocytotoxicity assays, the sera from rabbits inoculated with either adult worm purified membranes, or the 32000 mol. wt antigen partially purified from adult worm membranes, mediated levels of schistosomula killing as high as those obtained with sera from chronically infected mice. These rabbit antisera also promoted eosinophil adherence and killing of newly transformed schistosomula, but lung stage parasites, despite binding the anti‐membrane antiserum, were refractory to both humoral and cellular cytotoxicity. The significance of antigenic identity is discussed in relation to the concept of concomitant i