The serum gamma-globulin fraction from patients with allergic bronchial asthma (8/8), in contrast to the fraction from nonatopic control subjects (10/10), was found to inhibit the positive chronotropic action of the beta 2-selective adrenergic agonist, clenbuterol, on pyruvate- or lactate-treated cultured neonatal rat heart myocytes. No inhibition was exerted on the positive chronotropic response to prenalterol, which acted via beta 1-adrenergic receptors. The inhibitory effect of the asthmatic gamma-globulins was concentration dependent. It could nearly be abolished by immunoprecipitation with antihuman gamma-globulin and antihuman IgG, but not with antihuman IgM. This finding means that the inhibitory immunoglobulins of the patients with asthma were chiefly autoantibodies of the IgG isotype, capable of cross-reacting with chronotropic beta 2-adrenergic receptors on the cultured rat cardiomyocytes. These findings provide support for the notion that autoimmunity plays a role in the impairment of beta-adrenergic responsiveness in patients with allergic bronchial asthma.