Both in histologically proven (N = 3) and clinically diagnosed (N = 3) definite (peri)myocarditis in children, as well as in probable (N = 6) and possible inflammatory perimyocardial disease (N = 31), antibodies directed against the sarcolemma and myolemma (AMLAs) were demonstrated. In the acute phase AMLAs of the IgM class and complement fixation prevailed in the serum and the biopsy specimens. Complement dependent cytolysis by the patients' sera in the acute phase may simulate in vitro what could also be operative in vivo: a secondary immunopathogenesis due to cytolytic antimembrane antibodies.