Intrapleural instillations of recombinant interleukin 2 (RIL-2) were performed in 11 patients with malignant pleurisy due to lung cancer. Kinetic studies on RIL-2 concentration in the pleural effusion and serum revealed relatively long-term maintenance of detectable levels of RIL-2 (over 24 h in the pleural effusion and over 8 h in the serum). Clinically, pleural effusions and cancer cells in the effusions disappeared in 9 of the 11 patients 4 to 10 days after the start of the treatment. Lymphokine-activated killer cells were induced in the effusions of responders who exhibited the disappearance of pleural effusion and cancer cells from the effusion, but not in those of the nonresponders. This induction of lymphokine-activated killer cells may result in the disappearance of cancer cells and pleural effusions. Cytological examination of pleural effusions revealed increases of lymphoblasts, immunoblastic large lymphocytes, and eosinophiles in number and proportion in the responder, although such a phenomenon could not be observed in the nonresponders. Main and frequent side effects of intrapleural instillations of RIL-2 were fever up to 39°C, transient increase of pleural effusion, and eosinophilia. No serious side effect was encountered in our experience.