Experimental autoimmune myocarditis could be produced in mice by repeated injection of syngeneic heart extract together with Klebsiella pneumoniae O3 lipopolysaccharide (KO3 LPS) as a powerful adjuvant. Histological changes in the cardiac lesions were characterized by infiltration with mononuclear cells in the myocardium, degeneration and loss of myocardial fibers, and replacement of granulation tissues. No such cardiac lesions were produced in mice receiving injections of heart extract alone or KO3 LPS alone. Development of the autoantibody and the delayed type-hypersensitivity (DTH) against syngeneic heart extract was found in mice immunized repeatedly with the mixture of heart extract and KO3 LPS. Moreover, definite cardiac lesions were produced in normal recipient mice by transfer of sensitized spleen cells from hyperimmunized mice. Therefore, it was suggested that those cardiac lesions were caused by the autoimmune mechanism. Our methodology provided a new experimental murine model for autoimmune myocarditis.