AS LONG AGO as 1912, Howell suggested that L the active element in tissue extracts favoring coagulation is a phospholipid of the nature of cephalin. Robinson and Poole in 1956 observed that the addition of rat chylomicra to plasma from which the fat particles and platelets had been removed by high-speed centrifugation greatly enhanced the quantity of thrombin found in a thrombin generation test. Chylomicra contain a substance resembling phosphatidy ethanolamine in its Chromatographie behavior. It is believed that phosphatidyl ethanolamine or serine is probably the phosphatide of the chylomicra which is active in the coagulation tests. O'Brien demonstrated that the cephalins can, to varying extents, imitate the thromboplastic action of platelets. They may be the active principle in platelets responsible for the accelerated clotting of the blood. Axelrod, Beichenthal, and Brodie determined in 3 normal individuals the amount of phosphatidyl ethanolamine and serine in plasma and red blood cells