The sera from A.SW/SnJ mice infected with Coxsackievirus B3 (CB3) were tested on normal mouse tissue by indirect immunofluorescence. Heart-reactive antibodies were found. Absorption studies with organ extracts showed some of these autoantibodies to be heart-specific. Additional antibodies were crossreactive with skeletal muscle and kidney. These findings suggest a role for autoimmunity in the pathogenesis of murine CB3-induced myocarditis. This study establishes an animal model for the study of the humoral autoimmune response in human viral myocarditis.