A simple assay method of heparin cofactor II (HC II) activity is described. The procedure is based on the following principle: Antithrombin III (AT III) in plasma is inactivated by addition of an IgG fraction of goat serum after immunization of the animals against human AT III. Complete inactivation of AT III could be shown by absence of an anti Xa-effect of heparinized plasma treated with this antibody. Thrombin was only partially inhibited after inactivation of AT III. The characteristics of this inhibition were typical for the action of HC II. This method was applied for an assay of HC II activity. After optimizing of the method practical application in clinical routine screening was carried out. A diminution of HC II was observed in liver cirrhosis and in DIC but not in AT III deficiency. In 15 out of 269 cases of recurrent DVT there were HC II activities below 70% of normal. In 4 out of these patients activities of HC II were repeatedly between 44% and 52%. In arterial obstructive disease there was an HC II activity of less than 60% in 18 out of 583 patients and in 11 of them the HC II levels were repeatedly between 45% and 54%.