The novel endothelin (ET) receptor antagonists BE-18257A and BE-18257B were isolated from the fermentation products of Streptomyces misakiensis. The above-mentioned compounds inhibited [125 I] ET-1 binding to ET A receptors (selective for ET-1) on porcine aortic vascular smooth muscle cells (VSMCs) with IC 50 values of 1.4 and 0.47 μM, respectively.[125 I] ET-1 binding to ET B receptors (nonselective to ET isopeptides) in cerebellar membranes was not inhibited by either of these compounds even at 100 μM. The synthesized analogue BQ-123 induced extremely potent inhibition of [125 I] ET-1 binding to ET A receptors (IC 50 of 7.3 nM), but it barely inhibited [125 I] ET-1 binding to ET B receptors (IC 50 of 18 μM) and binding of various other peptides to their receptors. BQ-123 shifted the concentration-response curve for ET-1 toward the right in porcine isolated coronary arteries, indicative of competitive antagonism for the ET A receptor. However, there was a small amount of BQ-123-insensitive vasocontraction that paralleled the incomplete inhibition of [125 I] ET-1 binding in the membrane of the vascular smooth muscle layer. These data suggest that the artery contracts via both ET A and ET B receptors and that BQ-123 selectively inhibits ET A-mediated contraction. Furthermore, BQ-123 revealed large tissue and species differences in the distribution of ET A receptors. Thus, the potent ET A antagonist BQ-123 should be useful in clarifying the (patho) physiological roles of ET A receptors.