Cathepsin L was purified from the liver of a higher primate, the baboon (Papio ursinus), largely in a single-chain form and in the form of proteolytically active complexes with an endogenous cystatin. This mimics the situation found in both human and sheep livers. Both forms of cathepsin L were active at physiological pH. Physicochemical characterization and N-terminal amino sequencing of baboon cathepsin L showed a close relationship with the human enzyme. Cystatins with characteristics similar to those found for stefins A and B could also be purified from baboon livers. Proteolytically active, SDS-stable complexes could be shown to form in vitro with the molecules characterized as stefin B, but not with stefin A type cystatins. The non-inhibitory complexes could be shown to require less cysteine for activation than free cathepsin L and this, together with the above result, might indicate that a sulfhydryl interchange mechanism is responsible for the formation of covalent, non-inhibitory complexes.