SUMMARYThe levels and activities of endogenous IL-6 in malignant pleura! effusions due to lung cancer before and during daily intrapleural instillations of reeombinant IL-2 were examined by enzyme immunoassay and bioassay using an IL-6-dependenl murine hybridoma cell line MH60.BSF2. Before therapy, malignant pieural effusions contained various levels of IL-6. Daily intrapteural instillation of recombinani lL-2 for treatment of malignant pleurisy resulted in significant augmentation of the levels and activities of IL-6 in the pleura! effusions. On fractionation of the pieural effusion by chromatography. one major peak ofmateria! with a mo!, wi of 24 k D showed lL-6 activity. In contrast, no significant level of tumour necrosis factor-alpha or IL-I/J was delectable in pieural effusions before or during local IL-2 therapy. These data suggest that IL-2 is an important regulatory factor of secondary IL-6 producUoti.