Monoclonal antibodies against S. mansoni were produced by fusion of splenic lymphocytes from S. mansoni infected rats and P3-X63-Ag8 BALB/ cells. In vitro and in vivo studies of the biological activities of these antibodies led to identification of Ig[immunoglobulin]E antibodies with high reaginic activity and antibodies which, in a complement dependent or eosinophil dependent system, have a marked cytotoxicity for schistosomula in vitro. This methodology seems to open new perspectives for the study of antibody function in immunity against parasites and for the isolation of the corresponding target antigens.