A systematic study on the location and intensity of the fibrinolytic activity in more than 500 samples of human arteries and veins from 50 routine necropsies and 35 blood vessel biopsies was performed. Data were obtained for an overall comparison of the fibrinolytic activity along and across the walls of human blood vessels by the use of a standardized fibrin slide technique. Arteries generally showed little or no fibrinolytic activity in the intima and media but strong activity in the adventitia. Veins showed a comparable strong fibrinolytic activity in the external layer of loose connective tissue. Fibrinolytic activity in the venous intima, media and adventitia was generally weaker but varied greatly according to position in the body. Veins situated in the lower parts of the body had less fibrinolytic activity than the veins at the upper levels. Fibrinolytic activity was found to be related to the endothelium of the vasa vasorum and/or of the main lumen of the vascular wall. Increased fibrinolytic activity was observed in arteries and veins in cases of sudden death, vasogenic shock, cerebral hemorrhage and cirrhosis. Decreased fibrinolytic activity was encountered in blood vessels in cases of endotoxin shock, hyaline membrane disease and a case of Waterhouse-Friderichsen syndrome.