Precursor forms of lysosomal cathepsins B, H and L in the hepatic endoplasmic lumen were identified as having a molecular weight of 39-, 41-, and 39-kDa, respectively, by immunoblotting analysis. The proenzymes were then concentrated by applying the microsomal contents to a concanavalin A-Sepharose chromatography. The concanavalin A-adsorbed fractions containing the proenzymes showed no appreciable activities of cathepsins B, H and L. When those fractions were incubated at pH 3.0, the enzymatic activities markedly increased. Immunoblotting analysis showed that after 36 hr incubation the proenzymes disappeared and the mature enzymes increased. Thus the proenzymes were processed to the mature enzymes under acidic conditions of pH 3.0. The marked increased of enzymatic activities and the conversion of the proenzymes to the mature forms were completely blocked with pepstatin which is a potent inhibitor of aspartic proteinases. The results strongly suggested that a processing proteinase for procathepsins B, H, and L might be cathepsin D, a major lysosomal aspartic proteinase. Indeed, lysosomal cathepsin D could convert the immunoaffinitypurified microsomal procathepsin B to the mature enzyme in vitro. Therefore, procathepsins B, H, and L seem to be firstly synthesized as the enzymatically inactive forms in endoplasmic reticulum and may successively be converted into the active forms by cathepsin D in lysosomal compartments.