Abstract A comparative study of Lee and White 1 whole blood clotting times, recalcification times of activated platelet-rich plasma and citrated blood, and partial thromboplastin times of activated plasma and whole blood was carried out on heparinized patients being treated for thromboembolic disorders. The recalcification times were found to be sensitive and reliable monitors of heparin-induced hypocoagulability. The partial thromboplastin times of activated plasma, however, were reliable only within the lower part of the therapeutic range as determined by the Lee and White 1 clotting time. The latter technique was time consuming and the partial thromboplastin time of activated whole blood at room temperature proved unsatisfactory. Of the 2 recalcification time techniques, that with activated citrated blood is considered to be the method of choice for the control of heparin therapy as the result is obtained more rapidly.