Mouse cytomegalovirus (MCMV) infection induces persisting myocarditis in the susceptible BALB/c strain. Autoantibodies to cardiac myosin are produced in both susceptible BALB/c and resistant C57BL/6 mice following MCMV infection. These affinity-purified anti-cardiac myosin antibodies cross-react with MCMV protein(s). The polypeptides of CMV which share immunological cross-reactivity with the 200,000 MW polypeptide, the heavy chain of myosin, were viral polypeptides of 83,000, 94,000 and 116,000 MW recognized by BALB/c post-infection sera and polypeptides of 66,000 and 94,000 MW recognized by C57BL/6 post-infection sera. Passive transfer of anti-cardiac myosin antibodies from Day 56 post-infection sera of the BALB/c strain induced inflammation and necrosis of the myocardium of uninfected BALB/c recipients. This late immune sera contains autoantibodies specific for the cardiac isoform of myosin. Furthermore, immunization with cardiac myosin induced myocarditis and high titres of cardiac myosin antibodies in uninfected mice of the susceptible BALB/c strain only. However, antibodies to myosin elicited in cardiac myosin-immunized BALB/c mice did not cross-react with MCMV by ELISA. We suggest that virus infection may modulate the immune recognition of the common-epitope(s) shared between MCMV protein(s) and the heavy chain of myosin. Of particular interest is the possibility that molecular mimicry of CMV with cardiac myosin may contribute to the pathogenesis of autoimmune myocarditis following virus infection.