The extrinsic coagulation activity assay (EXCA) is a new thrombin generation test for the tissue factor pathway of coagulation. The EXCA was performed with 10 parts citrated plasma of different contents of fibrinogen. One part tissue factor, 250 mmol/l CaCl2, generating about 1 IU/ml thrombin within 1 min (37°C). After 0–30 min 2.5 mol/l arginine (pH 8.6) Generated thrombin was detected by addition of CHG-Ala-Arg-pNA and measurement of ▵A/t. The EXCA is dependent on factors 10% of the factor VII norm in the sample achieves 70–80% of the thrombin generation norm. The EXCA is not dependent on factors VIII, IX, XI and XII. Even in antithrombin III-deficient plasma, a phase of thrombin inhibition appears after the thrombin peak. Supplemented purified fibrinogen resulted in decreased thrombin generation in the important. Fibrinogen seems to act as antithrombin I; thrombin might be entrapped in the nascent fibrin, The EXCA is suitable to diagnose the level of extrinsic factors in patient plasma.