Cytotoxic activities of regional lymph node lymphocytes (RLNL) and peripheral blood lymphocytes (PBL) of 49 primary lung cancer patients who were subjected to surgical resection were examined by 4 h 51Cr release assay. PBL showed significantly lower cytotoxicity against autologous tumor cells than against K562 and QG-56. On the other hand, RLNL exhibited the same level of cytotoxicity against autologous tumor cells as PBL, although the cytotoxicities against K562 and QG-56 were low. Cytotoxicity of RLNL against autologous tumor cells exhibited a significant degree of depression with the advance of stage, T and N factors. Cytotoxicity of PBL did not significantly change as the stage progressed. When both PBL and RLNL were cultured with purified interleukin-2 (p-IL2) in vitro, their cytotoxic activities were markedly augmented and the cytotoxicities could not be diminished by the treatment with anti-Leu-7 + anti-Leu-11b + C'. These facts indicate that the augmented cytotoxicity may be due to lymphokine activated killer (LAK) cells.