Although the lysosomal cysteine proteinase cathepsin B is alkaline pH-labile, active, stable high molecular mass forms have been reported previously from the culture medium of human and murine mammary tumor explants and the sputum of patients with purulent bronchiectasis. A similar, catalytically active, high molecular mass form of recombinant human cathepsin B produced in yeast has now been found to represent a noncovalent complex between the 30-kDa single chain enzyme and its 6-kDa propeptide formed during autocatalytic maturation of the proenzyme (see accompanying article; Mach, L., Mort, J. S., and Glössl, J. (1994) J. Biol. Chem. 269, 13030-13035). Incubation of the complex under acidic conditions resulted in dissociation and degradation of the inhibitory propeptide leading to increased enzymatic activity, as also observed for partially purified cathepsin B isoenzymes from purulent sputum and mammary tumor explant media. The stabilization of the processed proteinase as a noncovalent complex with its proregion provides an important mechanism whereby extracellular cathepsin B can lie dormant until regional acidification mediates its activity.