The affinity (Ka) of human coagulation Factor Xa for thrombin-treated (to stimulate the release reaction) platelets has been determined to be 3 to 4 x 10(10) M-1 by equilibrium binding studies using 125I-labeled Xa. The binding of Factor Xa to platelets results in an increase of 300,000-fold in the apparent enzymatic activity of Xa in the conversion of prothrombin to thrombin. The activity of platelet surface Xa is approximately 15-fold greater than that observed with optimum concentrations of bovine Factor V and phospholipids in place of platelets. Ca2+ is required for the Xa-platelet interaction; the optimum concentration is 2.5 mM. Related coagulation factors, including Factor X, Factor IXa, diisopropylphosphoryl Factor Xa, and prothrombin do not complete with Factor Xa for the Xa binding sites. The rate of thrombim formation at saturating amounts of Xa is directly proportional to the number of platelets from 1 x 10(7) to 5 x 10(8) platelets/ml. Factor Xa bound to platelets is not inactivated by antithrombin III. An antibody that inhibits both human and bovine coagulation Factor V activity blocks both Xa binding to released platelets and the rapid thrombin formation associated with this binding, suggesting that Factor V from platelets is involved in the Xa-platelet interaction.