An amidolytic assay employing the chromogenic substrate S 2337 (Kabi Diagnostica) was used to assay factor X in 35 healthy controls and in 100 outpatients receiving oral anticoagulant therapy. This method correlated well with a coagulation assay of factor X in the control group (r = 0.88). When compared with two routine tests for the control of anticoagulant theray (Thrombotest and prothrombin ratio) good correlation was obtained between the methods, r = 0.84 and r = -0.74 respectively. These results suggest that a chromogenic substrate assay for factor X might be a suitable method for the maintenance control of oral anticoagulant therapy.