We recently reported that regional lymph node lymphocytes (RLNL) from patients with primary lung cancer were in a more highly activated state than peripheral blood lymphocytes (PBL) when the activation-related molecules were studied by FACS analysis. To identify whether or not RLNL had the ability to respond to autologous tumor cells (AT), in the present study a mixed lymphocyte tumor cell reaction (MLTR) either with or without recombinant interleukin 2 (rlL-2) was performed in 41 cases with primary lung cancer. Significant proliferative responses to AT were found in RLNL from 20 of 41 cases (48.8%) without IL-2, and in 23 of 41 cases (56.1%) with IL-2. On the other hand, such responses were observed in the PBL from 8 of 30 cases (26.7%) without IL-2, and from 11 of 30 cases (36.7%) with IL-2. No significant correlation between MLTR and such clinical factors as tumor size, metastasis to lymph node, histology and stage of the disease was found. To further analyze the anti-AT response of RLNL, the cytokine production of RLNL was investigated after stimulation by AT. An increase in interferon-gamma (IFN-gamma) production was observed in RLNL with a positive reaction of MLTR, while no such increase was found in PBL. Finally, to elucidate whether the expression of MHC class II molecules was a key point in MLTR, tumor cells in primary lesions were examined for the expression of MHC class II by immunohistochemical staining, and the blocking assay of MLTR was performed with anti-MHC class II monoclonal antibody. Data suggested that there was a positive correlation between MHC class II expression of the tumor and MLTR and that MLTR were partially blocked by anti-MHC class II monoclonal antibody. These results demonstrated that RLNL were in a more highly activated state against AT than were PBL, and this finding is considered to be helpful in enhancing our understanding of the role of RLNL in lung cancer patients.