The role of recombinant granulocyte-macrophage colony-stimulating factor (GM-CSF) in augmentation of lymphokine-activated killer (LAK) cell induction by interleukin-2 (IL-2) from pleural cavity mononuclear cells (PCMNCs) was examined in sixteen patients with resectable primary lung cancer not associated with malignant effusion. None of the patients had received any anticancer therapy prior to this study. Incubation of PCMNCs of patients without malignant effusion with GM-CSF for 4 days in the presence of IL-2 resulted in a significant increase in LAK activity against natural killer-resistant Daudi cells. This result was obtained by using the 4 h 51Cr-release assay. PCMNCs and blood mononuclear cells (BMNCs) were harvested simultaneously from pleural cavity lavage fluid and peripheral blood in lung cancer patients. The LAK activity developed from PCMNCs and BMNCs following incubation with IL-2 for 4 days, but the LAK activity from PCMNCs was significantly lower than that from BMNCs (P < 0.05). Incubation of PCMNCs with GM-CSF augmented the LAK activity from PCMNCs to a level as high as that from BMNCs. These results suggest that the combined use of GM-CSF with IL-2 may result in augmentation of LAK activity developed from PCMNCs of lung cancer patients without malignant effusion.