Lumen of aortic aneurysm is usually filled with parietal thrombus. Behaviour of the parietal thrombus is determined by the ratio of coagulation factors to factors of fibrinolytic system. Activity of some factors of coagulation and fibrinolysis in the parietal thrombus of aortic aneurysm was determined using coagulative, fibrinolytic and caseinolytic tests. Retracted, blood clot was a comparative material. Tissue factor activity in the parietal thrombus of the aneurysm was above threefold higher and antiheparin activity was nearly twice higher in comparison to the blood clot. Activity of plasminogen activators in the parietal thrombus was higher than in the blood clot. The parietal thrombus contained fourfold more of the plasminogen. Antiplasmin activity in the thrombus was above twofold lower than in the blood clot. High activity of the tissue factor and substances neutralizing heparin may intensify the thrombus growth. Yet the thrombotic tendency may be balanced by a high activity of plasminogen activators and plasminogen.