In myocarditis an antigen-specific immune response to cardiac epitopes has been demonstrated by several investigators. In 54 patients with histologically proven myocarditis, autoantibodies to cardiac tissue were observed in 73% of patients utilizing the indirect immunofluorescence test with human myocardium and adult heterologous cardiocytes. By immunoblot, 44% of sera from patients reacted with cardiac tissue. These antibodies were directed preferentially against proteins with the molecular weight of 43 up to 67 kDa. Three particular proteins were identified by the use of two-dimensional immunoblot and further characterized by amino acid sequence analysis. To characterize the epitopes recognized by the autoantibodies isoelectric focusing followed by SDS-PAGE was used to separate the complex mixture of proteins from human heart. Immunoblot analysis of antigens revealed proteins ranging from a size of 30-67 kDa at isoelectric points of 6.5-8.5 to be of particular interest. Five of these proteins have now been analyzed by N-terminal amino acid sequencing (Edman degradation). One was found to be creatine kinase and one was identified as a yet unknown protein. Three proteins were N-terminally blocked and were investigated further by enzymatical digestion, followed by separation of the usually complex peptide mixtures on HPLC. One of the peptides was found to be dihydrolipoamide dehydrogenase, a membrane enzyme, and one was identified as a sarcomere specific creatine kinase. Because these proteins are intracullularly located enzymes, their pathogenetic role as antigens for the autoantibodies remains to be elucidated further.