We previously reported that a cell-surface aminopeptidase, dipeptidyl peptidase IV, is expressed on extravillous trophoblasts (EVT) and suggested the involvement of its enzyme activity in EVT migration. In this study, we examined the expression of another cell-surface peptidase, carboxypeptidase-M (CP-M), at human embryo implantation sites, which catalyses biologically active peptides at extracellular sites. CP-M was immunohistochemically detected on syncytiotrophoblast, but not on cytotrophoblasts in floating chorionic villi (9-12 weeks of gestation). At villus-anchoring sites, CP-M was weakly detected on some EVT in the distal part of the cell column. CP-M was clearly expressed on EVT in the trophoblastic shells and in the maternal vessels. In the decidua, almost all interstitial trophoblasts expressed CP-M. Flow cytometry and RT-PCR showed that CP-M expression was induced on the outgrown EVT in primary villous explant culture. The CP-M induction on cultured EVT under 20% O(2) concentration was significantly higher than that under 1% O(2) concentration. In invasion assays, migration of JEG-3 cells, a CP-M-bearing human choriocarcinoma cell line, was significantly enhanced by an inhibitor of CP-M, DL-mercaptomethyl-3-guanidino-ethyltiopropanoic acid (MGTA). These findings indicate that CP-M is a differentiation-related molecule for human EVT and suggest that CP-M expression on EVT is partially regulated by tissue oxygen concentration.