To obtain a good animal model for polymyositis, we previously induced experimental autoimmune myositis (EAM) in Lewis rats by immunization with partially purified skeletal myosin. However, the nature of EAM-inducing antigen(s) in the partially purified myosin preparation remains unclear because it may contain several myositogenic antigens. In the present study, we further purified myosin and C-protein from partially purified myosin preparations and examined their EAM-inducing ability. It was revealed that immunization with both C-protein and purified myosin elicited EAM, which was essentially the same as that induced by partially purified myosin. However, their myositogenic ability was quite different. C-protein induced severe EAM of high histological grade and lesion frequency, whereas purified myosin induced only mild EAM. Immunohistochemical staining of C-protein-induced lesions demonstrated that muscle fiber-infiltrating cells were CD8beta+ T cells and macrophages and that CD4+ cells were mainly located in the endomysium and interfiber connective tissue. Collectively, these findings suggest that C-protein in the skeletal muscle is the major myositogenic antigen and induces inflammatory lesions mimicking those of human polymyositis.