The major excreted protein (MEP) purified from Kirsten-virus-transformed 3T3 fibroblasts and mature human cathepsin L were compared in respect to a number of catalytic criteria and found to be similar. The Mr of MEP is 39,000, whereas that of mature human cathepsin L is 30,000. Sequence data suggested that MEP could be a pro-form of mouse cathepsin L. Both enzymes acted on the synthetic substrate benzyloxycarbonyl-Phe-Arg-7-(4-methyl)coumarylamide with similar catalytic constants and acted optimally at pH 5.5. Both were rapidly inactivated by the active-site-directed inhibitors benzyloxycarbonyl-Phe-Phe-diazomethane and L-3-carboxy-trans-2,3-epoxypropionyl-leucylamido-(4-guanidin o)butane, and furthermore, 3H-labelled L-3-carboxy-trans-2,3-epoxypropionyl-leucylamido-(4-acetamid o)butane, which binds covalently to the heavy chain of mature cathepsin L, also bound to MEP. MEP autolyses rapidly at pH 3.0 to give lower-Mr (35,000 and 30,000) forms, but all forms react with the radiolabelled inhibitor. No autolysis occurred above pH 5.0. MEP hydrolysed azocasein at pH 5.0, demonstrating that it is capable of hydrolysing protein substrates without autolytic activation. Unlike mature forms of cathepsin L, MEP is stable, but not active, at neutral pH. The present work shows that cathepsin L can be secreted as a higher-Mr precursor that is stable in extracellular fluids but only active where local pH values fall below 6.0. These results suggest that the extra N-terminal peptide on MEP is not an activation peptide, but is a regulatory peptide affecting the pH-stability and activity of mouse cathepsin L.