Thromboxane A2 receptors of human astrocytoma cells and rabbit platelets were characterized by a binding of a new potent and selective thromboxane A2 antagonist, [3H]ONO NT-126. [3H]ONO NT-126 bound with a high affinity to membranes derived from astrocytoma cells and platelets. In astrocytoma cells, the nonspecific binding determined by 1 microM S-145 was observed in high percentage to total binding. Scatchard plots of both membranes were linear, expressing a single binding site with the Kd of 0.17 nM and Bmax of 46.4 fmol/mg protein in astrocytoma cell membranes and with the Kd of 0.28 nM and Bmax of 1585 fmol/mg protein in platelet membranes, respectively. [3H]ONO NT-126 binding was displaced most potently by ONO NT-126 among other thromboxane A2 analogues. [3H]ONO NT-126 binding was displaced by ONO NT-126, S-145, SQ29548, STA2 and U46619 with Ki values (nM) of 0.21, 1.14, 7.05, 23.5 and 235 in astrocytoma cell membranes and 0.23, 5.55, 259, 9.25 and 43.7 in platelet membranes, respectively. The characteristics of TXA2 receptors apparently differ in rabbit platelets and human astrocytoma cells.