Tumor-induced immunosuppression remains one of the major obstacles to many potentially effective cancer therapies and vaccines. Host interleukin (IL)-23 suppresses the immune response during tumor initiation, growth, and metastases, and neutralization of IL-23 causes IL-12–dependent antitumor effects. Here, we report that combining agonistic anti-CD40 monoclonal antibodies (mAb) to drive IL-12 production and anti–IL-23 mAbs to counter the tumor promoting effects of IL-23 has greater antitumor activity than either agent alone. This increased antitumor efficacy was observed in several experimental and spontaneous lung metastases models as well as in models of de novo carcinogenesis. The combination effects were dependent on host IL-12, perforin, IFN-γ, natural killer, and/or T cells and independent of host B cells and IFN-αβ sensitivity. Interestingly, in the experimental lung metastases tumor models, we observed that intracellular IL-23 production was specifically restricted to MHC-II^hiCD11c⁺CD11b⁺ cells. Furthermore, an increase in proportion of these IL-23–producing cells was detected only in tumor models where IL-23 neutralization was therapeutic. Overall, these data suggest the clinical potential of using anti-CD40 (push) and anti–IL-23 mAbs (pull) to tip the IL-12/23 balance in established tumors. Cancer Res; 74(9); 2412–21. ©2014 AACR.