To clarify whether apoptosis is involved in the injury processes induced by autoantibodyagainst cardiac β_1-adrenoceptor,we investigated the biological and apoptotic effects of antibodies on culturedneonatal rat cardiomyocytes.Wistar rats were immunized with peptides corresponding to the second extra-cellular loop of the β_1-adrenoceptor to induce the production of anti-β_1-adrenoceptor antibodies in the sera.Immunoglobulin(Ig)G in the sera was detected using synthetic antigen enzyme-linked immunosorbentassay and purified using the diethylaminoethyl cellulose ion exchange technique.Apoptosis of cardiomyo-cytes was evaluated using agarose gel electrophoresis and flow cytometry.Our results showed that thepositive serum IgG greatly increased the beating rates of cardiomyocytes and showed an“agonist-like”activity.Furthermore,positive serum IgG induced cardiomyocyte apoptosis after treatment with β_1-adrenoceptor overstimulation for 48h.The effects of monoclonal antibody against β_-adrenoceptor werealso found to be similar to those of positive serum IgG.It was suggested that the autoantibody could inducecardiomyocyte apoptosis by excessive stimulation of β_1-adrenoceptor. To clarify whether apoptosis is involved in the injury processes induced by autoantibodyagainst cardiac β_1-adrenoceptor, we investigated the biological and apoptotic effects of antibodies on cultured neonatal rat cardiomyocytes. Wistar rats were immunized with peptides corresponding to the second extra-cellular loop of the β_1- adrenoceptor to induce the production of anti-β_1-adrenoceptor antibodies in the sera. Immunoglobulin (Ig) G in the sera was detected using synthetic antigen enzyme-linked immunosorbent assay and purified using the diethylaminoethyl cellulose ion exchange technique. Apoptosis of cardiomyo-cytes was evaluated as using agarose gel electrophoresis and flow cytometry. Our results showed that thepositive serum IgG significantly increased the beating rates of cardiomyocytes and showed an “agonist-like” activity.Furthermore, positive serum IgG induced cardiomyocyte apoptosis after treatment with β_1-adrenoceptor overstimulation for 48h.The effects of monoclonal antibody against β_-a It was suggested that the autoantibody could inducecardiomyocyte apoptosis by excessive stimulation of β_1-adrenoceptor.