Postinfection sera from A.CA/SnJ A.SW/SnJ, B10.S/SgSf, and B10.PL/SgSf mouse strains which varied in their susceptibility to Coxsackievirus B3-induced immunopathology were suspected to contain autoantibodies against cardiac tissue. These sera were used to identify the target myocardial autoantigen(s). Sera pools were made during the peak of the early, virus-induced myocarditis at 5 and 7 days and during the peak of the late, immunopathic phase of myocarditis at Days 15 and 21 after infection. Only the A.CA/SnJ and A.SW/SnJ strains which develop the immunopathic heart disease had heart-specific autoantibodies as determined by indirect immunofluorescence. This panel of sera pools was then tested against solubilized extracts from whole heart and skeletal muscles. Results from Western immunoblotting analyses demonstrated that antibodies to myosin were a prominent feature in the sera of strains which developed immunopathic myocarditis. The immunopathic sera pools were subsequently assayed against low-salt, high-salt, and a number of detergent extracts of heart and skeletal muscle. Anti-myosin was still the most notable reactivity, even though other autoantigens were detected. Absorption with cardiac myosin removed the vast majority of heart reactivity from the pooled sera derived from the A.CA/SnJ and A.SW/SnJ strains as determined within the limitations of the immunofluorescent and immunochemical assays. Both sarcolemmal and A-band staining patterns were abolished by the cardiac myosin absorption. These studies suggest that myosin is one of the major autoantigens in Coxsackievirus B3-induced autoimmune myocarditis.