Autoprothrombin II activity develops when prothrombin preparations of human origin are activated with purified thrombin at pH 8.2. Early during the activation an inhibitor seems to form. Concentrates of the autoprothrombin II can replace serum in the thromboplastin generation test provided platelets are used in the test, but not if a soy bean phosphatide is used. Dogs were given Coumadin in doses that lowered their own autoprothrombin concentration practically to zero. Then while continuing with use of the drug some purified autoprothrombin II was infused intravenously. This was tolerated very well. The autoprothrombin II concentration stayed at normal for 7 hr. In 24 hr none remained. The infusion was also followed by a shortening of the whole blood clotting time.