A cDNA clone encoding human procathepsin B was expressed at a high level in Escherichia coli using a T7 polymerase expression system, resulting in the formation of insoluble cytoplasmic protein aggregates (inclusion bodies). The recombinant product was solubilized and renatured by refolding and reoxidation. The proenzyme was subsequently processed with pepsin to produce an enzymically active enzyme. By systematic variation of the parameters influencing the folding, formation of disulphide bonds, and processing of procathepsin B to the catalytically active mature form, a simple renaturation procedure was designed, allowing the production of about 3 mg purified active cathepsin B/l E. coli culture broth. The enzyme obtained in this way consists of a single chain and, as a consequence of pepsin treatment, possesses a three-amino-acid extension at its N-terminus. The enzyme has similar kinetic and immunological properties to native human cathepsin B.