Abstract
Thrombin inhibitors can be demonstrated directly after electrophoresis of plasma or serum in agarose gel if the gel contains a small amount of fibrinogen and is afterwards sprayed with a thrombin solution. The heparin cofactor activity can be demonstrated analogously if the gel is sprayed first with a heparin solution and then with a thrombin solution. A thrombin inhibitor of human plasma and serum, identified as antithrombin III', was completely separated electrophoretically from the heparin cofactor. Antithrombin III was shown to occur in higher concentration in plasma than in serum.