In guinea-pigs, myocarditis was induced under experimental conditions by immunizing the animals with rabbit skeletal muscle myosin-beta. The salient histopathological features were foci of perivascular lymphonononuclear aggregates, necrosis, and degeneration of myocardial cells. Antimyosin-beta antibodies in the immune complexes were demonstrated in the sera of the guinea-pigs. An immune-complex-mediated tissue-injury is proposed in the pathogenesis of myocarditis.