The activity of endogenous colony-stimulating factor (CSF) in malignant pleural effusions of lung cancer patients before and during daily intrapleural infusions of recombinant interleukin-2 (IL-2) was measured quantitatively by colony-forming bioassay and radioimmunoassay (RIA). Before therapy, malignant pleural effusions had various levels of CSF activities, and this CSF activity was neutralized almost completely by anti-M-CSF antibody. RIA also showed that the effusions contained various amounts of M-CSF. Daily intrapleural infusion of recombinant IL-2 caused significant increase in the CSF activities and M-CSF levels in pleural effusions. These results indicate that in vivo treatment with IL-2 induces production of endogenous M-CSF.