Abstract: An extract of adult Schistosome mansoni worms, obtained by incubation in 0.15 M sodium chloride-sodium phosphate buffer (pH 6.8), contains protein, carbohydrates, and nucleic acid and/or by-products of the latter component and resolves into four major fractions by gel chromatography in G-100 and G-200 Sephadex columns. Immunodiffusion tests with rabbit anti-total extract serum reveal three precipitation lines corresponding to fractions I and II, and one with III or IV. Rabbits immunized with this total extract are found to be totally or partially (at least 77%) resistant to a challenge infection. The saline extract antigenic material is an effective vaccine for the treatment and immunization of schistosomiasis and other schistosome infections.