Retinoids are well recognized as promising antitumor agents in humans. However, there have only been a few reports about the effect of retinoids in canine cancers. To investigate the antitumor effect of retinoids on mast cell tumors (MCT), inhibitory effect on cell growth and induction of apoptosis were examined in vitro. Although sensitivity of these cells differed among the cells, the growth of three MCT cell lines (CoMS, CM-MC and VI-MC) were inhibited dose dependently when they were treated with retinoids. FACS analysis of PI-stained nuclei revealed an apoptotic fraction in CM-MC cells about 30% when treated with retinoids, while those of control cells were less than 5%. Caspase-3 activation was observed after retinoid treatment in CM-MC cells. This was confirmed by inhibiting the retinoid-induced apoptosis using the pan-caspase inhibitor, ZVAD-FMK. Both retinoid receptors, RARs and RXRs, were detected by immunoprecipitation followed by western blot analysis in all the three MCT cells. These data suggests that retinoids inhibit the growth of MCTs partly through apoptosis, and this growth inhibition by retinoids may be mediated by RARs and RXRs. We conclude that retinoid may be a potential adjunctive chemotherapeutic agent for the treatment of canine MCT.