T cell-deprived mice acutely infected with S. mansoni suffer microvesicular hepatocyte damage which is not seen in infected, immunological intact animals. A cationic fraction (CEF6) of the PBS-soluble portion of S. mansoni eggs (SEA) induces antibodies which, on passive transfer, prevent hepatocyte damage. CEF6 contains 2 antigens, ω1 and α1, and has also been shown to be a useful serodiagnostic reagent. This paper describes the purification and characterization of the 2 antigens present in CEF6. ω1 is a monomeric glycoprotein with a pI > 9·0 and a molecular weight of 31 kDa. α1 consists of two immunologically cross-reactive dimers, 41 and 36 kDa in non-reducing conditions, each of which consists of one unique and one common glycoprotein subcomponent. In ELISA with mouse and human infection sera ω1 is shown to be S. mansoni specific and is better able to distinguish S. mansoni infections from other schistosome infections than are unfractionated SEA, CEF6 or α1. Passive transfer of monospecific anti-ω1 sera into S. mansoni infected, T cell-deprived mice completely prevented the occurrence of microvesicular hepatocyte damage in these animals. Monospecific anti-α1 serum had no hepatoprotective capacity.