Several antigenic preparations derived from Schistosoma mansoni schistosomula were compared as immunogens in proliferation assays utilizing peripheral blood mononuclear cells (PBMN) from patients with chronic intestinal schistosomiasis mansoni. Living and fixed schistosomula, either irradiated or nonirradiated, and freshly transformed or 24 hr cultured worms were found to induce very similar proliferative responses of patients' PBMN. Two schistosomulum surface membrane preparations were also compared. Membrane vesicles from schistosomula were found to elicit mean proliferative responses similar to those stimulated by a CaCl2-extracted tegument preparation and to do so at lower total protein concentrations. However, the responses to membrane vesicles were somewhat more variable. When saline extracts from schistosomular bodies submitted to these two treatments were used as stimulants in parallel PBMN cultures it was found that vesiculation more effectively removed antigens involved in the stimulation of PBMN. Broken vesicles were less stimulatory than intact ones. This suggests that the manner in which membrane antigens are presented to antigen processing cells is critical. Incubation of schistosomula at 37°C, but not 4°C in medium containing 10% fetal calf serum from 3 to 48 hr decreased the capacity of the membrane to produce vesicles when treated with high salt solution.