The Action of Plasmin on Fibrin and Fibrinogen in Blood.1

Summary. 1. Solutions of plasmin prepared from post‐mortem blood, split purified preparations of fibrinogen and fibrin at nearly the same rate. 2. In the blood plasmin is adsorbed to fibrin. The inhibitory effect of antiplasmin on fibrinogenolysis is therefore much greater that on fibrinolysis. In normal blood, fibrinogen was not affected by plasmin, while fibrin was rapidly split. 3. After death varying amounts of antiplasmin were found in blood. Correspondingly varying relationships between the rates of fibrinogendysis and fibrinolysis were observed. 4. The strong and specific adsorption of plasmin to fibrin and its high affinity for this substrate in blood is evidence of the specific adaption of plasmin to fibrin. 5. The identity of fibrinolytic enzymes prepared from blood in widely different ways and the action of these enzymes on different proteins are discussed. The significance of the fibrinolytic activities occurring under physiological conditions in blood is stressed.