Antisarcolemmal (ASAs) and in particular antimyolemmal antibodies (AMLAs) are a serologic hallmark of inflammatory heart muscle disease and its sequelae. Since they may also occur to a much lesser incidence with increasing age, it was examined whether they also possess properties of "natural antibodies". As natural antibodies, AMLAs and ASAs have specificity for conserved structures on the membrane. They possess cross-reactivity and increase with age. In contrast to natural antibodies, however, they occur most frequently after viral stimulation, and are more often of the IgG- and IgA- than of the IgM-isotype and fix complement in the acute stage of the disease. They also exhibit cytolytic and cytotoxic properties when incubated in vitro with isolated heart muscle cells. In addition, antigenic mimicry has been demonstrated to be operative, since they are cross-reactive to viral proteins.