Spontaneously beating cardiomyocytes of newborn rats were exposed in culture to the gamma globulin fraction from the blood serum of patients with allergic asthma and dilated cardiomyopathy. The gamma globulin fraction of the asthmatics inhibited in a dose-dependent fashion the positive chronotropic response of lactate or pyruvate-treated myocytes to clenbuterol, an adrenergic agonist that acted via stimulation of beta 2 adrenoceptors. The gamma globulin fraction of the cardiomyopathic patients increased the rate of beating. This effect was stereoselectively inhibited by the beta blocker (-)-propranolol, indicating an involvement of beta adrenoceptors. The serum gamma globulin fraction from healthy persons had no effect on the beating rate. The effects of both the asthmatic and cardiomyopathic gamma globulin fractions were abolished by immunoprecipitation with antihuman gamma globulin and antihuman IgG. The chronotropic effects of the gamma globulin fraction may be attributed to the presence of autoantibodies to beta-adrenergic receptors, more specifically beta 2 receptors in the case of the asthmatics and beta 1 receptors in the case of the cardiomyopathic patients.