Abstract Targeting negative regulators downstream of the TCR represents a novel strategy to improve cancer immunotherapy. Two proteins that serve as critical inhibitory regulators downstream of the TCR are diacylglycerol kinase ζ (DGKζ), a regulator of Ras and PKC-θ signaling, and Casitas b-lineage proto-oncogene b (Cbl-b), an E3 ubiquitin ligase that predominantly regulates PI(3)K signaling. We sought to compare the signaling and functional effects that result from deletion of DGKζ, Cbl-b, or both (double knockout) in T cells and to evaluate tumor responses generated in a clinically relevant orthotopic pancreatic tumor model. We found that whereas deletion of Cbl-b primarily served to enhance NF-κB signaling, deletion of DGKζ enhanced TCR-mediated signal transduction downstream of Ras/Erk and NF-κB. Deletion of DGKζ or Cbl-b comparably enhanced CD8+ T cell functional responses, such as proliferation, production of IFN-γ, and generation of granzyme B when compared with wild type T cells. Double-knockout T cells demonstrated enhanced function above that observed with single-knockout T cells after weak, but not strong, stimulation. Deletion of DGKζ, but not Cbl-b, however, resulted in significant increases in numbers of activated (CD44hi) CD8+ T cells in both nontreated and tumor-bearing mice. DGKζ-deficient mice also had enhanced control of pancreatic tumor cell growth compared with Cbl-b–deficient mice. This represents a direct comparison between mice of these genotypes and suggests that T cell immunotherapies may be better improved by targeting TCR signaling molecules that are regulated by DGKζ as opposed to molecules regulated by Cbl-b.