ABSTRACT To the Editor:— Certain aspects of the article "Effect of Bishydroxycoumarin (Dicumarol) on Clotting Time of Whole Blood" by Mayer and Connell in The Journal, June 30, page 806, merit further discussion. Until the appearance of this paper, there were serious questions about the rationale of Dicumarol usage. The general opinion had been that it did not affect significantly the clotting time of whole blood. Previous attempts to show changes in clotting time had given contradictory or inconclusive results. Accordingly, the question persisted, "If Dicumarol doesn't change the clotting time in the dosage given, how can one be sure it is an anticoagulant?" Because this question went unanswered, the argument for Dicumarol was weakened, and I believe that as a result many physicians were reluctant to use this drug. The details were discussed by me in a recent review article (Am. J. M. Sc.229:534 [May] 1955).The report