Background. Impaired T-cell function has been noted in tumor-infiltrating lymphocytes (TIL). Recently, loss of function was found to be associated with modifications in T-cell receptor complex (TCR)-mediated signaling. A common feature is loss or reduced expression levels of the signaling chain, TCRζ. We evaluated whether loss of function in TIL and tumor-associated lymphocytes (TAL) from patients with ovarian cancer is associated with changes in TCRζ expression, and which factors can cause these defects. Methods. TIL and TAL were isolated from multiple patients and evaluated for their proliferative capacity by stimulation with a polyclonal stimulus. In addition, expression of TCRζ and CD3ε was evaluated in fresh TIL and TAL by the Western blot technique. Finally, various conditions within a tumor environment were tested for their effect on TCRζ and CD3ε. Results. TIL, but not TAL, were significantly impaired in their proliferative response, even when both populations were derived from the same patient (P < .05). Reduced proliferation levels were associated with loss of expression of TCRζ but not of CD3ε. Exposure of normal T cells to relative ischemia or heat shock, or culture in medium without IL-2, did not significantly reduce expression of TCRζ compared with CD3ε. However, coculture of T cells with tumor-derived macrophages or tumor-derived factors led to a selective loss of TCRζ compared with CD3ε (P < .05). Further analysis suggested that oxides such as hydrogen peroxide secreted by macrophages may be responsible for loss of TCRζ and high molecular weight factors secreted by certain tumors. Conclusions. TIL but not TAL show impaired T-cell function, which is associated with loss of TCRζ. In addition to macrophages secreting oxides, loss of TCRζ may be caused by tumor-derived soluble factors.