The direct identification of beta adrenoceptors in endothelial cell cultures has not been possible until the advent of a new beta-adrenergic radioligand, [125I]iodocyanopindolol ([125I]ICYP). Using [125I]ICYP, we report the successful identification of a beta adrenoceptor in cultured bovine aortic endothelial cells. At 37 degrees C, specific binding is saturable, stable and reversible. There is a single class of binding sites (21,500 +/- 2,900 sites/cell) with an equilibrium dissociation constant (Kd) of 109 +/- 26 pM. The rate constant of association, k1, is 1.22 X 10(9) M-1 min-1 and of dissociation, k-1, is 0.01 min-1. Binding studies on monolayers of endothelial cells grown in microtiter plates yield similar data (Kd = 53 +/- 9 pM, Bmax = 20,000 +/- 1,900 sites/cell). Stereoselectivity of binding for the (-)-isomer is demonstrable for both agonists and antagonists. A series of adrenergic agonists competes with [125I]ICYP for binding with an order of potency suggesting beta 2 subselectivity ; isoproterenol (0.73 microM) greater than epinephrine (15 microM) greater than norepinephrine (71 microM). Furthermore, the beta 2 inhibitor butoxamine is more potent than the beta 1 inhibitor practolol (7.7 microM vs 22 microM respectively). The GTP analogue, Gpp(NH)p, reduces isoproterenol affinity to 1.9 microM and increases the Hill coefficient from 0.62-0.90.