The effect of the proteolytic cleavage of plasminogen activator inhibitor type 1 (PAI-1) by human neutrophil elastase (HNE) on fibrinolysis was investigated. HNE cleaved active PAI-1 and produced low molecular weight forms of inactive PAI-1, as previously reported. Latent PAI-1 was resistant to HNE treatment. Vitronectin (VN) partially protected the cleavage. NH2-terminal sequence analysis indicated that the cleavage site was Val355-Ser356 (P4-P3). The effects of PAI-1 cleavage by HNE on clot lysis was studied in a purified system. Clot lysis time without PAI-1 was 20.0 +/- 5.0 minutes and was prolonged to 86.7 +/- 2.9 minutes by 68 nmol/L of PAI-1. It was shortened when HNE (from 0.6 nmol/L to 80 nmol/L) was added and returned to the value obtained without PAI-1 by 80 nmol/L of HNE (20.0 +/- 5.8 minutes). However, in the absence of PAI-1, elastase did not enhance clot lysis at all. Euglobulin clot lysis time was also shortened after HNE treatment. The cleavage and inactivation of PAI-1 by HNE was shown to be a novel pathway to enhance fibrinolysis.