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The coagulation time is a measurement of the intrinsic power of the blood to convert fibrinogen to fibrin. It is an empirical test no matter how performed, and therefore in order to be reliable requires that the test be done on venous blood under strictly controlled conditions. A recommended procedure is outlined in detail. The coagulation time is prolonged in hemophilia, hypoprothrombinemia, afibrinogenemia and heparinemia. In hemophilia, the coagulation time theoretically is a measure of the severity of the disease but practically is of limited value since the coagulation time may be within normal limits in some patients; the prothrombin consumed in the coagulation of hemophilic blood is therefore a better guide for diagnosis. The coagulation time in hypoprothrombinemia is relatively little prolonged until a drastic reduction occurs. The test is therefore of no value for establishing a hemorrhagic condition in hypoprothrombinemia. In afibrinogenemia the blood is incoagulable. A small amount of fibrinogen restores the coagulation time to normal. The presence of heparin increases the coagulation time. The test is therefore useful in controlling the therapeutic action of this drug. The senior author, in making a survey of the literature on hemorrhagic diseases in preparation of his monograph, was impressed by the significant and diverse contributions which Dr. George R. Minot made to this field of medicine. We feel honored to contribute this study to the collection of papers offered as a fitting tribute to Dr. Minot, who has so successfully and productively combined science and clinical medicine.