Human CD8+ T cells activated and expanded by TCR cross-linking and high-dose IL-2 acquire potent cytolytic ability against tumors and are a promising approach for immunotherapy of malignant diseases. We have recently reported that in vitro killing by these activated cells, which share phenotypic and functional characteristics with NK cells, is mediated principally by NKG2D. NKG2D is a surface receptor that is expressed by all NK cells and transmits an activating signal via the DAP10 adaptor molecule. Using stable RNA interference induced by lentiviral transduction, we show that NKG2D is required for cytolysis of tumor cells, including autologous tumor cells from patients with ovarian cancer. We also demonstrated that NKG2D is required for in vivo antitumor activity. Furthermore, both activated and expanded CD8+ T cells and NK cells use DAP10. In addition, direct killing was partially dependent on the DAP12 signaling pathway. This requirement by activated and expanded CD8+ T cells for DAP12, and hence stimulus from a putative DAP12-partnered activating surface receptor, persisted when assayed by anti-NKG2D Ab-mediated redirected cytolysis. These studies demonstrated the importance of NKG2D, DAP10, and DAP12 in human effector cell function.