Agonistic monoclonal antibodies (mAbs) directed against the co signaling molecule CD137 (4-1BB) elicit potent anti-tumor immunity in mice. This enhanced anti-tumor immunity has traditionally been thought to result from the ability of the Fab portion of anti-CD137 mAb to function as a CD137L analogue. Although engagement of CD137 by anti-CD137 mAbs has the potential to cross-link the Fc fragments, mediating Fc engagement of low to moderate affinity Fc gamma receptors (FcγR), the relative import of such Fc-FcγR interactions in mediating CD137 associated anti-tumor immunity is unknown. In order to address this knowledge gap, we studied the ability of rat anti-mouse CD137 mAb (2A) to mediate the anti-tumor response against the EL4E7 lymphoma in WT and FcγR-/- strains. We hypothesized that Fc interactions through activating FcγRs would enhance the anti-tumor immune response elicited by anti-CD137 mAb. 2A treated Fcγ chain deficient mice have improved anti-tumor immunity against EL4E7 lymphoma relative to WT mice, which can be completely recapitulated in FcγRIII-/- mice. We tested the hypothesis using the MC38 colon carcinoma, but did not find the same enhanced anti-tumor immune response in the FcγRIII-/- mice as in the EL4E7 lymphoma. Kinetic experiments to analyze the immune response in the tumor bearing 2A treated FcγRIII-/- mice identified an increase in splenic CD8β+ T cell and dendritic cell (DC) populations compared to WT mice and IgG controls. There was a significant increase in the number of DCs expressing CD40, CD80, and CD86 molecules in the 2A treated FcγRIII-/- mice suggesting the potential for more effective antigen presentation compared to WT mice. The increased numbers of splenic CD8β+ T cell and dendritic cell (DC) populations in the 2A treated FcγRIII-/- mice significantly correlated with the tumor volume on Day 16. Collectively, these data suggest FcγRIII ligation negatively regulates anti-CD137 mAb stimulation of DCs with a secondary increase in CD8β+ T cells involved in the anti-tumor immune response. Our results demonstrate an unexpected inhibitory role for FcγRIII in the anti-tumor function of anti-CD137 mAb in the EL4E7 lymphoma, and underscore the need to consider antibody isotype when engineering therapeutic mAbs.