The course of infection with Schistosoma mansoni was determined in B cell deficient mice by means of a schistosomule lung recovery assay 6 days after infection or by determination of the adult worm burden 7 weeks after infection. The intensity of infection was not significantly different from that in age- and sex-matched intact controls. B cell deficiency was demonstrated by absence of surface immunoglobulin-bearing cells in the spleen and by absence of B cell areas in the lymphoid follicles of the spleen and mesenteric lymph nodes. In addition, B cell deficient mice infected for 7 weeks with S. Mansoni were unable to form anti-schistosome antibodies detectable by the Cercarienhüllenreaktion. A normal granulomatous response, however, was observed around schistosome eggs. Pretreatment with BCG suppressed infection with S. mansoni comparably in intact and B cell deficient mice. A marked depletion of eosinophils occurred in the schistosome egg granuloma of all BCG treated mice.