Infection with type 3 of the group B Coxsackieviruses (CB3) sometimes leads to the development of myocarditis in humans. Circumstantial evidence in the form of heart-reactive antibodies in these cases of human myocarditis suggests that the later phases of the disease may be due to autoimmunization. Since human myocarditis is a relatively rare sequel to infection with CB3 virus, we propose that it reflects a genetic predisposition in some individuals. To investigate this possibility we established an experimental murine model of viral myocarditis. By testing a large number of strains of inbred mice infected with CB3 we found that a few strains developed an ongoing myocarditis characterized by diffuse interstitial mononuclear infiltration and by the production of heart-specific IgG autoantibodies. These antibodies reacted with myocardial sarcolemma and myofibrils as well as with muscle striations. The principal myocardial autoantigen, identified by means of postinfectious sera of mice with heart-specific autoantibodies, was found to be the cardiac isoform of myosin. Immunization of susceptible mice with cardiac myosin stimulated the production of heart-specific antibodies reactive with both cardiac muscle striations and sarcolemma, accompanied by mononuclear infiltration of the myocardium. From these results we infer that cardiac myosin is an autoantigen capable of inducing postinfectious myocarditis in genetically predisposed individuals.