Polymorphonuclear leucocytes (PMN) are important in the resolution of human thrombi, with u-PA as a key player. We have shown that the u-PA activity of PMN depends on the presence of plasma; the study presented here provides an explanation for that requirement. Here we show that PMN degraded scu-PA and also tcu-PA, t-PA and plasmin, resulting in loss of fibrinolytic activity. Plasma protected against this degradation; alpha1-antitrypsin was identified as a protective factor. Purified human neutrophil elastase mirrored the effects of PMN, again neutralized by plasma inhibitors. These findings illustrate the dual role of PMN in the breakdown of thrombi, in that they contribute both u-PA, which lyses fibrin, and other proteases, including elastase, which can cleave fibrin and plasminogen activators/plasmin. Similarly, plasma can potentiate fibrinolysis by neutralization of PMN elastase, in addition to direct inhibition of fibrinolytic proteases. Our previous studies show that PMN in thrombi are mostly pro-fibrinolytic; the anti-fibrinolytic role defined here may be important in other pathologies where fibrin persists.