The method of testing for antithrombin in the blood is at present very difficult, as it requires the preparation of a pure fibrinogen containing no prothrombin, that is to say, which does not clot upon the addition of calcium, and also the preparation of a pure thrombin which can be made from fibrin. As a result the method is hardly adaptable to general clinical use. For some time I have employed a method which seems to meet this difficulty. For this purpose about 9 c.c. of blood are aspirated and put into I c.c. of I per cent. sodium oxalate. The blood is centrifugalized and the plasma siphoned off in the usual way. The plasma is then recalcified by adding 2, 3, 4 and 5 drops respectively of a 1/2 per cent. calcium chloride solution. In this way we ascertain the general coagulability of the plasma which is the composite of a number of factors,—prothrombin, fibrinogen and antithrombin, and we determine the optimal amount of calcium for this particular plasma. If we heat some of this plasma to 60° C., the prothrombin, as is well known, is destroyed and the fibrinogen is coagulated. After filtering off this coagulum, we have a plasma which contains antithrombin. The strength of this antithrombin may be ascertained for clinical purposes as follows: First from a normal case we prepare human plasma just as we prepared the oxalated plasma which is to be tested. Five drops of this plasma are put into five thoroughly cleansed vials. One of these serves as a control, to the second three drops of the normal antithrombin is added, to the third five drops of normal antithrombin; to the fourth three drops of the antithrombin that is to be tested, and to the fifth five drops of this antithrombin.