TWO COMPONENTS of the blood proteins, prothrombin and fibrinogen, are affected in the regeneration and destruction of tissue. Whipple and his associates1 showed that injury to hepatic tissue or hepatectomy will reduce the fibrinogen content of the blood. On the other hand, "injury or inflammation of tissues is the most powerful stimulus to fibrinogen production."2 Fibrinogen and prothrombin are essential elements required for the clotting of blood which occurs at the site of any injury. In a previous publication, it was shown that in cases of acute myocardial infarction the prothrombin time becomes shortened.3 As the injured area heals, the clotting time returns to normal. The present investigation dealt with another of the blood proteins, the plasma fibrinogen, in an effort to determine whether similar changes occur in such cases. MATERIALS AND METHODS Twenty-eight patients with acute myocardial infarction were studied. Plasma fibrinogen was determined for each