The coagulation time of the blood in hereditary hemophilia fluctuates in an irregular manner from day to day. Only very pronounced alterations are therefore of value as a guide to the effect of any particular method of treatment. The variations shown in Table I were observed in cases who were not subjected to any treatment. In many instances the changes observed are well beyond the error of the method which was used. Five cubic centimeters of blood were withdrawn from the median basilic vein through a short oiled needle into two or more test-tubes, and the average interval of time required until coagulation had advanced sufficiently to allow of the complete inversion of the tubes without spilling the contents was taken as the coagulation time. The temperature was 37° C. Normal blood requires about 13 minutes to coagulate under these conditions. Parallel observations with another method showed that reliable results could be obtained with blood from skin puncture when certain details in the manner of collec...