Balb/c mice were immunized with homologous heart in complete Freund's adjuvant to induce autoimmune myocarditis. The myocarditis was characterized by lymphomononuclear infiltration, electrocardiographic abnormalities and antimuscle antibodies by indirect immunofluorescence. In this paper, we demonstrate that the IgG present in autoimmune myocarditis mice is able to bind to beta-adrenoreceptors of the heart and also induce a biological effect inhibiting the contractile action of exogenous norepinephrine. Auto-immune IgG inhibited the binding of (3H)-dyhidroalprenolol to a beta-adrenergic receptor of purified myocardial membranes behaving as non-competitive inhibitor. This IgG also exerted a non-competitive inhibition upon the mechanical effect of exogenous norepinephrine. The recognition appears to be organ specific, because the autoimmune myocarditis IgG did not bind to beta-lymphocyte, lung and fat adrenoreceptors. The autoimmune IgG inhibited the stimulatory action of isoproterenol on cAMP levels, behaving as a beta-adrenergic antagonist.