Tumor-infiltrating lymphocytes (TILs) are often seen in non-small cell lung cancers (NSCCs). Their functional role in the pathogenesis of lung cancer is unknown. The authors studied TILs in 27 patients with NSCC and determined the following: (1) the immunologic phenotype as defined by monoclonal antibodies against various surface markers, (2) activation state as indicated by interleukin-2 (IL-2) receptor expression and the kinetics of proliferation response to IL-2, and (3) the ability to develop lymphokine-activated killer (LAK) type cytotoxicity against both natural killer (NK)-resistant tumor cell targets (M14) and fresh autologous tumor cells. The authors' results show TILs from NSCCs to be a heterogeneous population composed of T-cells, B-cells, monocytes, and NK cells in frequencies similar to those found in peripheral blood lymphocytes (PBLs). TILs demonstrated increased IL-2 receptor expression and a more rapid proliferative response to IL-2 than PBLs, implying activation of TILs by the tumor milieu. Finally, TILs generated cytotoxicity against NK-sensitive (K562) and NK-resistant (M14) cell line targets consistently after in vitro treatment with IL-2 but were less consistent in their ability to lyse fresh autologous tumor cells and less effective than PBL LAK cells in lysing all targets. Comparison with LAK cells generated from normal volunteers suggests that decreased killing of autologous tumor cells only partially results from an inherent resistance to lysis by fresh NSCC targets. It appears, therefore, that tumor cells taken from NSCCs are not readily killed by the immune cells that infiltrate the tumor stroma and that this failure does not result from nonspecific immune deficiency in TILs.