YM155 and its combination with alemtuzumab significantly prolonged the survival of MET-1 leukemia-bearing mice. The animals treated with YM155 alone, alemtuzumab alone, and the combination of YM155 with alemtuzumab had significantly prolonged survivals when compared with the PBS control group (P < .0001). The combination of YM155 with alemtuzumab significantly prolonged the survival of leukemia-bearing mice when compared with YM155 alone (P < .001) or alemtuzumab alone (P < .05). *Two tumor-unrelated deaths excluded from the analysis are described in the “Materials and methods.”