Single intravenous administration of IL-1 to intact white rats resulted in a two-phase change in fibrinolysis: short-time activation and subsequent inhibition. Activity of the tissue activator of plasminogen (TAP) increased in tissues of the experimental rats. This fact is indirect evidence of release the inhibitor of TAP from the vascular endothelium of these organs in blood flow. A 4 hour incubation of IL-1 with blood caused no inhibition of fibrinolysis. No significant changes in the haemostasis system were found.