In previous studies we have shown that combined chemo-immunotherapy of L1210 leukemia with IL-12 and doxorubicin results in striking anti-tumor effects producing 100% of long-term survivors. In this study we investigated the efficacy of a combination of IL-12 and mitoxantrone in murine L1210 leukemia. Mice inoculated with 1x105 leukemia cells were treated with a single dose of mitoxantrone and seven daily doses of IL-12, and were daily observed for survival. Treatment with IL-12 or mitoxantrone given alone produced moderate anti-leukemic effects. However, combination of both drugs resulted in a significant prolongation of mouse survival time. Importantly, there were almost 50% of long-term (>60 days) survivors among the mice treated with both agents. This therapeutic effect was completely abrogated by sub-lethal, whole-body X-irradiation, and significantly reduced after macrophage depletion.