It appears that lymph node metastases are more frequent in lung cancer than in other cancers because of impaired defensive mechanisms in the regional lymph nodes. However, little is known about the immunologic function of regional lymph node lymphocytes (RLNL) in patients with lung cancer. We have studied the immunologic properties of RLNL in comparison with peripheral blood lymphocytes (PBL). We measured the natural killer (NK) cell activity of RLNL and PBL in patients with lung cancer and found that the NK activity was significantly more depressed in the RLNL than in the PBL. In contrast, interleukin-2 (IL-2) production was markedly higher in the RLNL than in the PBL. The cytotoxic effect of RLNL in nonmetastatic lymph nodes on target cells (such as K562 cells) or PC-3 and PC-10 cells (NK-resistant, human lung cancer of adenocarcinoma and epidermoid carcinoma, respectively) was significantly enhanced byin vitroincubation with recombinant IL-2 (rIL-2). Furthermore, we clarified that both rIL-2 and OK-432, which is a biologic response modifier and IL-2 inducer as well, augmented the cytotoxicity of RLNL and that these effector cells were lymphokine-activated killer (LAK) cells. The depletion of lymphocyte subsets by pretreatment with specific monoclonal antibody showed that the LAK activity in RLNL was mediated by CD3+ and CD8+ cells, whereas the lymphocyte subsets contributing the LAK activity in PBL were CD3+ and CD16+ cells. It was concluded that a majority of the effector cells in RLNL were LAK cells of the cytotoxic T cell population.