In THE MORE than two decades since Campbell and Link¹ isolated bishydroxycoumarin as the hemorrhagic agent in spoiled sweet clover, numerous other coumarins and the related indandione derivatives have been developed and widely used as anticoagulants.² Studies of the effect of these oral anticoagulants on blood coagulation have indicated that most of these drugs depress not only prothrombin,³ but three other clotting activities as well—factors VII,⁴ IX,⁵ and X⁶—although evidence has been presented^7,8 that certain anticoagulant drugs differ qualitatively in their effect on the clotting mechanism. Furthermore, despite numerous isolated studies on individual drugs, the exact sequence in which the levels of various affected clotting factors decrease with coumarin or indandione therapy is not a matter of general agreement.⁹ A systematic study of the effect of commonly used coumarin and indandione drugs using specific assays for each of the four coagulation factors were the same as those usually observed during