The highly specific beta-adrenergic radioligand (-)[125I]iodocyanopindolol (ICYP) was used to characterize the beta-adrenergic receptor subtype present in human placenta. Binding of ICYP to membranes from human placenta was saturable with time and ligand concentration, of high affinity, and demonstrated appropriate stereoselectivity and agonist rank order of potency for binding to a beta-adrenergic receptor. From saturation binding curves, the KD and Bmax values for ICYP binding were 233 +/- 51 pM and 690 +/- 139 fmol/mg of proteins, respectively. Analysis of inhibition of ICYP binding by beta 1- and beta 2-selective adrenergic antagonists via Hofstee analysis resulted in linear plots, indicating the existence of a homogeneous population of beta-adrenergic receptors. From the resulting KI-values for the beta 1-selective drugs practolol (4.0 +/- 0.9 microM) and metoprolol (0.19 +/- 0.07 microM) and for the beta 2-selective drug ICI 118,551 (0.30 +/- 0.06 microM) it is concluded that the beta-adrenergic receptor in human placenta is of the beta 1-subtype. This is further supported by the fact that (-)-noradrenaline and (-)-adrenaline were equipotent in inhibiting ICYP binding.