Background Clinical application of adoptive T cell therapy has been hindered by an inability to generate adequate numbers of nontolerized, functionally active, tumor-specific T cells, which can persist in vivo. In order to address this, we evaluated the impact of interleukin (IL)-12 signaling during tumor-specific CD8+ T cell priming in terms of persistence and antitumor efficacy using an established B16 melanoma tumor adoptive therapy model. Study Design B6 mice were injected subcutaneously with B16 melanoma tumor cells. On day 12 of tumor growth, mice were preconditioned with cyclophosphamide (4mg dose, intraperitoneally), and 1 day later were treated by adoptive transfer of tumor-specific pmel-1 CD8+ T cells primed ex vivo 3 days earlier with both IL-12 and antigen (hGP10025-33 peptide) or antigen only. Tumors were measured biweekly, and infused donor T cells were analyzed for persistence, localization to the tumor, phenotype, and effector function. Results Adoptive transfer of tumor-specific CD8+ T cells primed with IL-12 was significantly more effective in reducing tumor burden in mice preconditioned with cyclophosphamide compared with transfer of T cells primed without IL-12. This enhanced antitumor response was associated with increased frequencies of infused T cells in the periphery and tumor as well as elevated expression of effector molecules including granzyme B and interferon-γ (IFNγ). Conclusions Our findings demonstrate that ex vivo priming of tumor-specific CD8+ T cells with IL-12 dramatically improves their in vivo persistence and therapeutic ability on transfer to tumor-bearing mice. These findings can be directly applied as novel clinical trial strategies.