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The immune response to the tumor can be enhanced by targeting costimulatory molecules on T cells. As the CD70-CD27 costimulatory axis plays an important role in the activation, survival, and differentiation of lymphocytes, we have examined the efficacy of agonistic anti-CD27 antibodies as monotherapies for established melanoma in a murine model. We show that this approach leads to a substantial reduction in the outgrowth of both experimental lung metastases and subcutaneous tumors. Anti-CD27 treatment supports the maintenance of tumor-specific CD8+ T cells within the tumor, reduces the frequency of FoxP3-expressing CD4+ T cells within tumors, and potentiates the ability of NK1.1+ and CD8+ tumor infiltrating cells to secrete IFNγ upon coculture with tumor cells. The enhanced effector function correlated with lower levels of PD-1 expression on CD8+ T cells from anti-CD27–treated mice. Despite the modulating effect of anti-CD27 on multiple cell types, only CD8+ T cells were absolutely required for tumor control. The CD4+ T cells were dispensable, whereas NK1.1+ cells were needed during early stages of tumor growth but not for the effectiveness of anti-CD27. Thus, CD27-mediated costimulation provides a potent boost to multiple aspects of the endogenous responses to tumor, and may be exploited to enhance tumor immunity.