The peptidase purified to homogeneity from Lactococcus lactis subsp. lactis biovar. diacetylactis ATCC 13675 was considered to be an aminotripeptidase (EC 3.4.11.4) from the results of substrate specificity. The K-m value showed a tendency to decrease with the number of alanine residues, but to increase with the number of glycine residues in the substrate tripeptide. The effects of divalent metal ions on enzyme activity were considerably different depending on the tripeptide used as a substrate. In the case of Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and Cd2+, there was apparent correlation between enzyme activities observed in the presence of metal ions and following metal ion replacement. The Zn2+-replaced enzyme showed almost the same K-m and k(cat) values as the native enzyme, suggesting the enzyme to be a zinc metallopeptidase. The K. of the divalent metal-replaced enzyme increased in the order of Co2+, Zn2+, and Mn2+. As a result of replacement with Co2+ an enzyme having 2.3-fold higher activity compared to the native enzyme for GGF as a substrate was obtained. Thus, the change in substrate specificity observed following metal replacement may suggest a highly specific interaction between the enzyme, metal and substrate, leading to the activity expression and stability of the tripeptidase.