The pathogenesis of myocardial cell injury in myocarditis and cardiomyopathy was investigated. The presence of viral genomes in the murine heart in experimental coxsackievirus B3 myocarditis was studied by Northern blotting analysis using a 32P-labeled cDNA probe from the 5' end sequence. The strongest signal of positive autoradiograms was always at about 7.4 kilobases, corresponding to the size of the complete genome of the virus. Successive infections with coxsackievirus and encephalomyocarditis (EMC) virus infection showed simultaneous acute myocarditis and healed myocarditis, and the results suggest that successive virus infections cause additional myocardial damage, and develop lesions similar to chronic myocarditis or dilated cardiomyopathy. Anti-heart auto-antibody, induced during EMC virus infection, reacted predominantly with myosin. Indium-111 antimyosin scintigraphy showed positive in some of the patients with cardiomyopathy, and the uptake was inversely correlated with left ventricular function. When mice were injected with antimyosin antibody, mouse immunoglobulin G was detected in hearts in the chronic stage of EMC virus myocarditis, in myocytes surrounding fibrosis and calcification, suggesting deposition of antimyosin antibody. Although further study is necessary to clarify the mechanism of uptake of antimyosin in cardiomyopathy, antimyosin antibody may accumulate in viable myocytes with ongoing degeneration as well as in necrosis.