Increased levels of both the cysteine protease, cathepsin L, and the serine protease, uPA (urokinase-type plasminogen activator), are present in solid tumors and are correlated with malignancy. uPA is released by tumor cells as an inactive single-chain proenzyme (pro-uPA) which has to be activated by proteolytic cleavage. We analyzed in detail the action of the cysteine protease, cathepsin L, on recombinant human pro-uPA. Enzymatic assays, SDS-PAGE and Western blot analysis revealed that cathepsin L is a potent activator of pro-uPA. As determined by N-terminal amino acid sequence analysis, activation of pro-uPA by cathepsin L is achieved by cleavage of the Lys158-Ile159 peptide bond, a common activation site of serine proteases such as plasmin and kallikrein. Similar to cathepsin B (Kobayashi et al., J. Biol. Chem. (1991) 266, 5147-5152) cleavage of pro-uPA by cathepsin L was most effective at acidic pH (molar ratio of cathepsin L to pro-uPA of 1:2,000). Nevertheless, even at pH 7.0, pro-uPA was activated by cathepsin L, although a 10-fold higher concentration of cathepsin L was required. As tumor cells may produce both pro-uPA and cathepsin L, implications for the activation of tumor cell-derived pro-uPA by cathepsin L may be considered. Different pathways of activation of pro-uPA in tumor tissues may coexist: (i) autocatalytic intrinsic activation of pro-uPA; (ii) activation by serine proteases (plasmin, kallikrein, Factor XIIa); and (iii) activation by cysteine proteases (cathepsin B and L).