At the 4th International Congress on Thrombosis and Hemostasis, an expert panel on Oral Anticoagulant Control was established jointly by the International Committee for Standardization in Hematology and by the International Committee on Thrombosis and Hemostasis. The panel was asked to make recommendations on standardization of methods and materials used for oral anticoagulant control so that a similar intensity of anticoagulant treatment might be generally used and optimal therapeutic levels of anticoagulation obtained in individual patients. The recommended test procedure for the prothrombin time test is detailed.