Summary. Several activated coagulation factors have been reported to enhance fibrinolysis by neutralizing plasminogen activator inhibitor type 1 (PAI-1) activity. We evaluated the physiological relevance of this mechanism using the euglobulin clot lysis time (ECLT) assay in the presence and absence of Ca2+, which is controlled by PAI-1 and mimics physiological thrombolysis. We found that the ECLT (18.5 ± 0.6 h) was shortened by Ca2+ (5 mm) (6.6 ± 0.1 h). A significant difference was observed in thrombin generation by the presence of Ca2+ in the euglobulin fraction. Prothrombin was almost fully converted to thrombin within 15 min in the presence of Ca2+, whereas essentially no conversion was observed without Ca2+. The presence of activated protein C (aPC) suppressed thrombin generation, and attenuated the shortening of ECLT in a dose-dependent manner, an effect enhanced by phospholipid and protein S. In the absence of Ca2+, aPC did not prolong the ECLT. After addition of biotin-labeled recombinant PAI-1 to the euglobulin fraction, PAI-1 was cleaved to lower molecular weight forms only in the presence of Ca2+. This cleavage did not occur in the presence of aPC, suggesting that thrombin was the catalyst for PAI-1 cleavage. The cleavage and inactivation of PAI-1 by generated thrombin is proposed to be responsible for the shortening of ECLT by Ca2+ and for coagulation-associated over-expression of fibrinolysis. Under such conditions, aPC appeared to suppress thrombin generation and to normalize highly activated fibrinolysis.