Clinical and experimental data suggest that autoimmunological mechanisms may play an important role in the pathogenesis of postmyocarditic cardiomyopathy. This may be due to the viral infection itself, or may be induced by persistence of the virus. Apart from the clinical signs of myocarditis, which are usually not specific, the diagnosis of acute myocarditis should be enabled by the histological examination of endomyocardial biopsies. In accordance with the Dallas criteria the histological diagnosis of acute myocarditis is defined by the presence of inflammatory cells in the myocardium associated with myocyte necrosis and degeneration of adjacent myocytes. This morphology, however, is only seen within the first 7 to 10 days of the acute stage of the disease. Later, most cases of clinically suspected acute myocarditis are histopathologically consistent with the diagnosis of "borderline myocarditis". Although endomyocardial biopsies have markedly improved diagnostic possibilities, the diagnosis of myocarditis by light microscopy has its limitations. Mainly the differentiation of infiltrating mononuclear cells from interstitial fibroblasts or pericytes is visually difficult. New immunohistochemical methods were therefore introduced to improve the specificity and sensitivity of the diagnosis. Using monoclonal antibodies against cell surface markers from lymphocytes (CD3, CD4, CD8) the identification, characterization and quantification of lymphocytic infiltrates in the myocardium is improved very significantly. Furthermore, the use of monoclonal antibodies against MHC-class-I and class-II-antigens provides further information about the immunological status of the myocardium. In conclusion, the use of these new immunohistological methods offers the possibility of establishing immunological criteria in addition to histological parameters.(ABSTRACT TRUNCATED AT 250 WORDS)