Although it is recognized that the number of cardiac β-adrenoceptors is reduced in human dilated cardiomyopathy, the mechanisms involved have not been defined. We examined the possible role of altered humoral immunity by comparing the effect of sera from patients with idiopathic dilated cardiomyopathy (n=20), ischemic or valvular heart disease (n=28), or controls with no known cardiac disease (n=18) on the binding of radioligands to cardiac β-receptors. The ability of sera from cardiomyopathic patients to inhibit the binding of [3H]dihydroalprenolol to rat cardiac membranes was significantly higher than that of the other two patient groups (40±5% at 50-fold serum dilution compared to 14±3% for the ischemic/valvular heart disease group, and 14±4% for the normal control group, p<0.001). A similar inhibition was exerted by IgG from cardiomyopathic patients. Only the number, not the affinity, of the β-receptors was decreased by cardiomyopathic sera. This decrease could be prevented by preincubating the sera with anti-human IgG, indicating the presence of autoantibodies. Furthermore, the sera were ineffective against cardiac α1-adrenoceptors and considerably less effective against lung β2-receptors. In addition to ligand binding inhibition, sera from cardiomyopathic patients could immunoprecipitate β-adrenoceptors quantitatively from solubilized cardiac membranes. Positive sera inhibited significantly isoproterenol-stimulated adenylate cyclase with no effect on basal or NaF-stimulated activities. These results document the presence in sera from patients with idiopathic dilated cardiomyopathy of autoantibodies directed against the cardiac β-adrenoceptor which may play an important role in the regulation of inotropic responsiveness to β-agonists.