As part of a study of the metabolic role of peptidases in Escherichia coli K-12, cell extracts were examined for the presence of three enzymes originally identified [Sussman, A. J., and Gilvarg, C. (1970), J. Biol. Chem. 245, 6518] in extracts of the lysine auxotroph ASO13 by virtue of their activity toward lysine homopolymers. It has now been shown that the activity ascribed to a Co2+-dependent dilysine-specific enzyme is a function of the strain K-12 dipeptidase DP, a metal-dependent enzyme active toward a variety of dipeptides, and that the activity ascribed to a trilysine-specific enzyme is a function of the strain K-12 tripeptidase TP, an aminopeptidase capable of hydrolyzing substrates in the series X-Gly-Gly, X-Gly-X, and X-Leu-Gly (where X is Leu or Met) but devoid of activity toward dipeptides. The third enzyme, an oligopeptidase not previously observed in strain K-12, was found to include among its substrates not only di- and trilysine but other di- and tripeptides that are hydrolyzed by the di-and tripeptidase as well as by aminopeptidases L and AP; the aminopeptidases, however, lack activity toward di- and trilysine. The absence of oligopeptidase activity from extracts of strain AJOO5, a "PEPTIDE-DEFICIENT MUTANT" derived from strain ASO13 by Sussman and Gilvarg, has been confirmed, and strain AJOO5 has been shown to contain all the other peptidases known to be present in strain K-12. Possible functions of the oligopeptidase are proposed on the basis of its observed activity in vitro and of the basis of its observed activity in vitro and of the differences between the growth responses of strains AJOO5 and ASO13 in various media. Some general aspects of peptide metabolism are discussed with emphasis on the use of peptidase-deficient mutants in the study of this problem, and methods that may prove helpful in the isolation of such mutants are suggested.