Abstract: (-) ³H dihydroalprenolol (³H DHA), a potent adrenergic antagonist, was used to identify binding sites in the erythrocyte membrane of Beijing duck. Binding of ³H DHA to the duck erythrocyte membrane exhibited characteristics expected of β-adrenoceptor. The binding was shown to be saturable, with high affinity and K_D value of 2.95 nM and B_max value of 102 fmol/mg protein. β-Adrenoceptor agonists and antagonists compete for these binding sites in a manner directly parallels to their relative activities for β-adrenoceptor. Thus, the binding is highly stereospecific with the (-) isomer of isoprenaline being two orders of magnitude more potent than the (+) isomer in competing for these sites. The order of potency for inhibition of binding by β-adrenergic agonists was found to be (-) isoprenaline > (-) norepinephrine= (-) epinephrine> (+) isoprenaline. The binding of ³H DHA to duck erythrocyte membrane was found to be rapid and reversible, with an association rate constant of 0.46 min^-1.nM^-1 and a dissociation rate constant of 0.42 min^-1. These results suggest that the binding sites in the erythrocyte membrane of the Beijing duck are β-adrenoccptor. In our exprements, the β-adrenoceptor of duck erythrocyte exhibited approximately the same affinity for norepinephrine and epinephrine, suggesting that β-adrenoceptor are dominant in the duck erythrocyte membrane. The density of β-adrenoceptor in the duck erythrocyte is lower than that in turkey erythrocyte. However, the membrane protein obtained per ml. duck blood may be five times as high as than that obtained per ml turkey blood. The nonspecific bindings of ³H DHA to the duck erythrocyte membrane was shown to be very low. In addition, the preparations of the erythrocyte mebrane keeps well in deep freezrer. Since duck is more popular than turkey in China, we suggest that duck erythrocyte be used in β-adrenoceptor studies in stead of turkey erythrocyte.