An IgM lambda class monoclonal antibody raised against membranes from rat dorsal root ganglia defines a novel antigenic determinant expressed by subpopulations of mammalian central and peripheral neurones. In the presence of complement the antibody is cytotoxic to mammalian neurones in vitro. The same antibody labels Trypanosoma cruzi, the protozoan responsible for Chagas' disease. Classes of mammalian neurones and cardiac muscle that are labelled by the antibody are known to degenerate in Chagas' disease. The common neuronal and trypanosomal antigens recognized by the antibody may therefore be important in pathogenic events underlying this disorder.