Abstract Tumor-resident CD8+ T cells display a quiescent effector/memory phenotype that is maintained in part by infiltrating CD4+ CD25+ Foxp3+ T-suppressor cells. Intratumoral delivery of IL-12, in contrast, can restore cytotoxic function to tumor-associated CD8+ T cells and induce the apoptotic death of T-suppressor cells. Depletion of CD8+ T cells from tumors before IL-12 treatment resulted in the abrogation of treatment-mediated T-suppressor cell apoptosis revealing a link between CD8+ T cell activation and T-suppressor elimination. Furthermore, IL-12 failed to induce T-suppressor cell loss in IFN-γ- or FasL-deficient mice demonstrating a requirement for IFN-γ and FasL in this process. Adoptive transfer of wild-type CD8+ T cells to FasL-knockout mice restored posttherapy T-suppressor cell elimination from tumors establishing that expression of FasL on CD8+ T cells was sufficient to promote T-suppressor cell death. IL-12 failed to induce FasL on T-effectors in IFN-γ-knockout mice demonstrating a requirement for IFN-γ in FasL up-regulation. Adoptive transfer of wild-type CD8+ T cells induced T-suppressor cell death in IFN-γ-knockout mice confirming that autocrine IFN-γ was sufficient for CD8+ T cell FasL expression. These findings reveal a mechanism by which cytotoxic T cells can abrogate regulatory cell activity.