Prothrombin is determined with the aid of a recently developed assay, based on the amidolysis of a chromogenic substrate. The assay proved to be reliable when it was compared with more conventional coagulation assays in the control of oral anticoagulant therapy, both in the therapeutic range and in a case of overdosage. As is the case in coagulation tests, heparin therapy remains a disturbing circumstance. The prothrombin concentration was measured (a) in the plasma of 50 long-term anticoagulated patients, and the results were compared with those obtained with a one-stage coagulation assay and with those obtained with Thrombotest® determinations, and (b) during vitamin K administration in the plasma of a patient with a severe intoxication of a vitamin K antagonist.