A peptidase from the cell wall fraction of Lactococcus lactis subsp. cremoris IMN-C12 has been purified to homogeneity by hydrophobic interaction chromatography, two steps of anion-exchange chromatography, and gel filtration. The molecular mass of the purified enzyme was estimated to be 72 kDa by gel filtration and 23 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme has a pI of 4.0, and it has the following N-terminal sequence from the 2nd to the 17th amino acid residues: -Arg-Leu-Arg-Arg-Leu-?-Val-Pro-Gly-Glu-Ileu-Val-Glu-Glu-Leu-Leu. The peptidase is most active at pH 5.8 and at 33 degrees C with trileucine as the substrate. Reducing agents such as dithiothreitol, beta-mercaptoethanol, and cysteine strongly stimulated enzyme activity, while p-chloromercuribenzoate had an inhibitory effect. Also, metal chelators lowered the peptidase activity, which could not be restored with Ca2+ and Mg2+. The divalent cations Cu2+, Zn2+, Fe2+, and Hg2+ completely inhibited peptidase activity. The peptidase is capable of hydrolyzing tripeptides and some dipeptides, with a preference for peptides containing leucine and with the highest activity towards the tripeptides Leu-Leu-Leu, Leu-Trp-Leu, and Ala-Leu-Leu, which were hydrolyzed with Kms of 0.37, 0.18, and 0.61 mM, respectively.