In this paper, the ability of human cardiac myosin to provoke autoimmune myocarditis was investigated. Myosin fractions were immunized into A.SW. mice, Lewis rats or Hartley guinea pigs. All of the immunized rats displayed overt symptoms of myocarditis and, in a few cases, died from it. The hearts of these rats were enlarged and discolored. Histologically, the muscles of the heart were characterized by remarkable cell infiltration, extensive myofiber necrosis and the appearance of polynuclear giant cells. Neither mice nor guinea pigs showed such disease profile. In this novel experimental model, the disease state was transferable by T lymphocytes. Thus, cardiac myosin was shown to provoke muscle cell damage through a T cell mediated autoimmune process.