Twenty-three patients (mean age 27 years, 15 male, 8 female) with hypertrophic cardiomyopathy (10 'obstructive', 13 'non-obstructive') mostly of the sporadic type were examined for anticardiac antibodies circulating in the peripheral blood or bound in the endomyocardial biopsy. Immunoglobulin deposition in the biopsies was found in 41% of cases of the IgG isotype. IgM and complement fixation occurred less frequently. Predominant findings in the patients sera were antimyolemmal (78%) and antifibril antibodies (43%). The patients' sera were not cytolytic except for one case indicating that neither a toxic serum factor nor antibody-mediated cardiocytolysis were operative to a significant degree in hypertrophic cardiomyopathy. The patient's lymphocytes did not demonstrate a cytotoxic effect either alone or in the presence of the autologous serum.