Protease Ci, a cytoplasmic metalloprotease in Escherichia coli, has been purified to apparent homogeneity by conventional chromatographic procedures using 125I-labeled oxidized insulin B-chain as a substrate. The purified enzyme behaves as a 54-kDa protein under both denaturing and nondenaturing conditions, suggesting that it consists of a single polypeptide chain. It is inhibited by metal-chelating agents, including o-phenanthroline and NaCN, but not by inhibitors of serine proteases or thiol-blocking agents. Furthermore, protease Ci was found to contain 1.1 mol of zinc per mol of the enzyme upon analysis by HR ICP mass spectroscopy. Thus, protease Ci must be a zinc metalloprotease. Among the polypeptides tested as substrates, oxidized insulin B-chain and glucagon are most rapidly hydrolyzed. Intact insulin is a much poorer substrate than oxidized insulin B-chain, even though the affinity of the enzyme to intact insulin is approximately 100-fold greater than that to the B-chain. Since unlabeled oxidized insulin A-chain is capable of inhibiting the hydrolysis of 125I-labeled insulin B-chain, it also appears to be a substrate. Protease Ci also degrades lysozyme and lactalbumin, although to a much lesser extent than oxidized insulin B-chain. However, it shows little or no activity against proteins larger than 15 kDa (e.g. ovalbumin and denatured bovine serum albumin). Hydrolysis of oxidized insulin B-chain followed by amino acid composition analyses of the cleavage products reveals that as many as 10 of its 29 peptide bonds are hydrolyzed by protease Ci. This ability to hydrolyze relatively small polypeptides suggests that protease Ci may catalyze the later steps in the pathway for intracellular protein breakdown.