Apoptosis inhibitor of macrophage (AIM) is initially reported to protect macrophages from apoptosis. In this study, we determined the effect of AIM on the macrophage-derived tumor, histiocytic sarcoma cell lines (HS) of dogs. Five HS and five other tumor cell lines were used. When recombinant canine AIM was applied to non-serum culture media, cell numbers of all the HS and two of other tumor cell lines decreased dose-dependently. The DNA fragmentation, TUNEL staining and flow cytometry tests revealed that AIM induced both of apoptosis and cell cycle arrest in the HS. Although AIM is known as an apoptosis inhibitor, these results suggest that a high dose of AIM could have an opposite function in HS and some tumor cell lines.