IL-6 has been characterized as a pleiotropic cytokine with multiple biologic activities, but its induction and role in asbestos diseases have not been studied. Asbestos fibers were found to stimulate IL-6 expression and secretion in pulmonary type II-like epithelial A549 cells as well as in normal human bronchial epithelial cells. IL-6 induction was dependent on the intracellular redox-oxidative state, since intracellular hydroxyl scavengers and N-acetylcysteine, a precursor of glutathione, abrogated IL-6 secretion by asbestos or H2O2. IL-6 induction paralleled increased DNA binding activity to the nuclear factor-kappa B (NF-kappa B)- and NF-IL-6-recognized sites in the IL-6 promoter. The NF-kappa B and NF-IL-6 DNA binding proteins were immunochemically characterized as a heterodimer p65/p50 and a homodimer C/EBP beta, respectively. Stimulation of DNA binding activity to the NF-kappa B and NF-IL-6 binding sites of the IL-6 promoter by asbestos or H2O2 were inhibited by tetramethylthiourea, a hydroxyl radical scavenger. The role of local IL-6 production in the pathophysiologic processes of fiber-induced lung disorders was examined. Although less active than fibroblast growth factor, human rIL-6 also stimulated lung fibroblast growth, as evidenced by increased [3H]thymidine incorporation. Furthermore, elevated IL-6 levels were found in bronchoalveolar lavage fluids from patients diagnosed with lung fibrosis and work-related histories of long term asbestos exposure. Taken together, the results suggest that asbestos-induced oxidative stress is involved in the activation of NF-kappa B and NF-IL-6 transcription factors, which recognize the IL-6 promoter. The resulting increase in IL-6 expression may be involved in both inflammatory and fibrotic processes in the lung.