Bestatin (Ubenimex) competitively inhibits many aminopeptidases, including B, N and leucine aminopeptidases. Ubenimex is a microbial metabolite and dipeptide with potential immunomodulatory and antitumor activities. Aminopeptidases has been implicated in the process of cell adhesion and invasion of tumor cells. Therefore, inhibiting aminopeptidases may partially attribute to the antitumor effect of ubenimex. This agent also activates T lymphocyte, macrophage, and bone marrow stem cell as well as stimulates the release of interleukin-1 and -2, thus further enhances its antitumor activity.