One of the problems of Dicumarol administration has been determination of the adequacy of therapy. A fundamental stage in thrombus formation in the circulation is platelet clumping. Since platelets are involved in both hemostasis and thrombosis, a study of changes in platelet function may prove useful. The effects of Dicumarol on platelet clumping and adhesiveness have, therefore, been studied and related to changes in other clotting mechanisms.Comparison of platelet function in control periods and during Dicumarol administration shows that this drug lowers the adhesive index and prolongs the platelet clumping time to a highly significant degree. The accuracy of estimates of these 2 platelet functions given by 3 other tests, prothrombin time, whole blood clotting time and plasma activity in the thromboplastin generation tests have been compared. By any criterion, degree of correlation, consistency of regression slopes, intercepts or scatter about regression lines, clotting time was the most efficient of the 3 tests for evaluting platelet function. The prothrombin time was the least efficient. Furthermore, hemorrhage from excessive dosage was closely related to the platelet clumping time and clotting time. It should be pointed out, however, that in about one-third of the patients the prothrombin time was as good an index of platelet clumping time as clotting time. While prothrombin times done with 2 different sources of thromboplastin may give congruous results in untreated patients, considerable discrepancies may appear during Dicumarol therapy.If prothrombin time is to be used as a guide to Dicumarol therapy, it is important to establish in each patient separately its reliability as a measure of the behavior of the clotting mechanism as a whole. For those in whom it seems reliable, an individual “therapeutic range” should be determined. For other cases, some other suitable test should be employed. It is emphasized that regulation of Dicumarol dosage is an individual problem and that only those tests shown to be reliable indices of safe effective modifications of clotting mechanism in the individual patient should be used.