Affinities of denopamine, a new cardiotonic agent, and several beta-adrenergic drugs for turkey erythrocyte membranes (TEM) and rat reticulocyte membranes (RRM) which contain homogeneous beta 1- and beta 2-receptors, respectively, were studied by receptor binding. The order of potencies of denopamine and several beta-adrenergic agonists in displacing 3H-dihydroalprenolol binding (Ki, nM) in TEM was isoproterenol (Iso, 27) greater than norepinephrine (Nor, 360) greater than epinephrine (Epi, 860) greater than dobutamine (DB, 1380) greater than denopamine (1540) greater than dopamine (DA, 49500). The order in RRM was Iso (7.3) greater than Epi (58) greater than DB (750) greater than Nor (1090) greater than denopamine (2300) greater than DA (26800). In the presence of GTP, competition curves for full agonists like Iso, Epi and Nor shifted to the low affinity side (Ki values increased by 300-500% in TEM and 200-460% in RRM), and the slopes were steepened in both membrane preparations. The Ki value for denopamine increased only in TEM (70%) and that in RRM was not influenced by GTP. This suggests that denopamine has an agonist property at the beta 1-receptor but not at the beta 2-receptor and that the intrinsic activity at the beta 1-receptor of the drug is lower than full agonists. Affinities of DB and DA for TEM were influenced by GTP as well as those for RRM, although the extent of the rightward shift was less than full agonists.