We have examined the antibody activity and nephritogenicity of anti-Engelbreth-Holm-Swarm (EHS) sarcoma antiserum in order to analyze the pathogenesis of the EHS nephropathy which has already been reported by us. An increased amount of urine protein was not recognized in rats injected with a large quantity of anti-EHS sarcoma antiserum. In addition, rats immunized with rabbit IgG before anti-EHS sarcoma antiserum injection developed no abnormal proteinuria, despite positive fluorescent staining for rat IgG as well as rabbit IgG in the glomeruli. From these results we concluded that nephritogenic antibodies could not be produced in rabbits by immunization with EHS sarcoma, which could induce the EHS nephropathy in an active model.