Aminopeptidase N (APN/CD13), a 150-kDa metalloproteinase, is a multifunctional cell surface aminopeptidase with ubiquitous expression. Recent studies have suggested that APN/CD13 plays an important role in tumor progression in several human malignancies. In the current study, we investigated the role of APN/CD13 in paclitaxel (PAC)-resistance of ovarian carcinoma (OVCA) cells. We first examined the correlation between APN/CD13 expression and IC50 values of PAC in a variety of OVCA cell lines. Next we investigated whether suppression of APN/CD13 using bestatin, an inhibitor of APN/CD13 activity or the siRNA technique influenced PAC-sensitivity in ES-2 cells, which highly express APN/CD13. Moreover, we investigated the effect of bestatin on peritoneal metastasis using nude mice. We found a negative correlation between APN/CD13 expression and chemosensitivity to PAC in various carcinoma cell lines. Subsequently, we found a significant increase in PAC-sensitivity of APN/CD13 expressing OVCA cells by suppression of this enzyme, using the addition of bestatin or the siRNA technique. Furthermore, in a peritoneal metastasis model using nude mice, combination treatment with PAC and bestatin caused a synergistic increase of survival time compared with PAC alone treatment. (mean survival time: 37.7 ± 7.0 s and 27.1 ± 6.6 days, respectively). The present findings showed that APN/CD13 may be involved in decreased sensitivity to PAC in OVCA cells and that the mechanism of this effect involves its enzyme activity at least in part. APN/CD13 may be a therapeutic target for the treatment of OVCA in combination with chemotherapy. © 2007 Wiley-Liss, Inc.