Through the kindness of Dr. J. H. Milstone we were able to perform clotting studies with purified bovine thrombokinase. Phenylmethyl sulfonylfluoride was found to inactivate bovine thrombin. Bovine thrombokinase, freed of traces of contaminating thrombin by treatment with phenylmethyl sulfonylfluoride, behaved in a manner similar to activated Stuart factor. Kinetic studies with this purified clotting factor and partially purified bovine proaccelerin supported the hypothesis that bovine proaccelerin is changed by the enzymatic action of thrombokinase to an agent which converts prothrombin to thrombin. The reaction between thrombokinase and bovine proaccelerin requires phospholipid and calcium and is blocked by soybean trypsin inhibitor. The similarities between human activated Stuart factor and bovine thrombokinase are apparent.