Cytotoxic T cells (CTLs) and natural killer cells play a major role in the immune response to Epstein-Barr virus (EBV) infection. In X-linked lymphoproliferative (XLP) disease, a severe immunodeficiency, immunodysregulatory phenomena are observed following EBV infection, suggesting that defects exist in these effector populations. The gene defective in XLP is SAP (signaling lymphocytic activation molecule [SLAM]–associated protein), an adaptor protein that mediates signals through SLAM and other immunoglobulin superfamily receptors including 2B4. We generated EBV-specific T-cell lines from controls and XLP patients and examined CTL function in response to different stimuli. We show that XLP patients can generate EBV–T-cell lines that are phenotypically similar to those from controls. XLP patient EBV–T-cell lines showed a significant decrease in interferon-gamma (IFN-γ) production in response to 2B4 and autologous EBV-transformed lymphoblastoid cell line (LCL) stimulation but not in response to SLAM. Furthermore, XLP EBV–T-cell lines demonstrated markedly decreased cytotoxic activity against autologous LCLs. By retroviral gene transfer of the SAP gene into XLP EBV–T-cell lines, we show reconstitution of IFN-γ production and of cytotoxic activity confirming SAP-dependent defects. These studies demonstrate that in XLP the lack of SAP affects specific signaling pathways resulting in severe disruption of CTL function.