A chemical method has been devised for the quantitative determination of plasma antithrombin. A concentrate of antithrombin has been prepared from plasma. It also contains heparin cofactor activity. The two distinct activities could not be separated and may quite possibly be the properties of a single substance. When the concentrate is combined with heparin the capacity of the concentrate to destroy thrombin is not increased but there is interference with the thrombin fibrinogen interaction. In a mixture of the concentrate and thrombin there is simultaneous destruction of antithrombin, and thrombin activities. Small amounts of the concentrate significantly prolong the clotting time of blood.