The binding characteristics of [3H]U46619, a tritiated thromboxane A2/prostaglandin H2 mimetic, were studied in pig aorta smooth muscle membranes. Binding was fast, saturable, selective and reversible. The KD values determined by kinetics, equilibrium binding and drug competition methods were 68, 42 and 53 nM, respectively. The Bmax was 87.8 fmol/mg protein. Specific binding was competitively displaced by thromboxane A2/prostaglandin H2 antagonists. Binding was also displaced by prostaglandins D2, E2 and F2 alpha with IC50 values of 8, 21 and 12 microM, respectively. U46619 contracted the rat and pig aorta smooth muscle, but the response of the latter was slower than that of the former. The antagonists prevented the U46619-induced contraction of rat aorta with the same rank order as that for the inhibition of ligand binding in pig aorta smooth muscle membranes. These results provide evidence that the putative thromboxane A2/prostaglandin H2 receptor in vascular smooth muscle membranes can be detected by a ligand binding technique.