A simple method for the quantitative assay of vascular plasminogen activator is described. From 24 chronic renal failure patients who were undergoing surgery for A/V fistular or shunt construction, small pieces of artery (mean 7.8 mg, range 3.0-18.5 mg) and vein (mean 4.5 mg, range 3.0-8.8 mg) were used. The fibrinolytic activity was assayed on a well-controlled fibrin plate by only distilled water immersion and incubation at 37 degrees C. Basal fibrinolytic activity was correlated with venous fibrinolytic activity (r = 0.773, p less than 0.0001) and with arterial fibrinolytic activity (r = 0.55, 0.005 less than p less than 0.01). The increment of fibrinolytic activity by venous occlusion was correlated to venous fibrinolytic activity (r = 0.849, p less than 0.0001) but not to arterial fibrinolytic activity (r = 0.34, 0.1 less than p less than 0.25).