Solid tumors are complex masses with a local microenvironment, or stroma, that supports tumor growth and progression. Among the diverse tumor-supporting stromal cells is a heterogeneous population of myeloid-derived cells. These cells are alternatively activated and contribute to the immunosuppressive environment of the tumor; overcoming their immunosuppressive effects may improve the efficacy of cancer immunotherapies. We recently found that engineering tumor-specific CD8+ T cells to secrete the inflammatory cytokine IL-12 improved their therapeutic efficacy in the B16 mouse model of established melanoma. Here, we report the mechanism underlying this finding. Surprisingly, direct binding of IL-12 to receptors on lymphocytes or NK cells was not required. Instead, IL-12 sensitized bone marrow–derived tumor stromal cells, including CD11b+F4/80hi macrophages, CD11b+MHCIIhiCD11chi dendritic cells, and CD11b+Gr-1hi myeloid–derived suppressor cells, causing them to enhance the effects of adoptively transferred CD8+ T cells. This reprogramming of myeloid-derived cells occurred partly through IFN-γ. Surprisingly, direct presentation of antigen to the transferred CD8+ T cells by tumor was not necessary; however, MHCI expression on host cells was essential for IL-12–mediated antitumor enhancements. These results are consistent with a model in which IL-12 enhances the ability of CD8+ T cells to collapse large vascularized tumors by triggering programmatic changes in otherwise suppressive antigen-presenting cells within tumors and support the use of IL-12 as part of immunotherapy for the treatment of solid tumors.