SUMMARY. Interleukin-6 (IL-6) is a pleiotropic cytokine, which, apart from its role in inflammation and the immune response, influences a variety of systems, including the respiratory system. IL-6 is recognized by two membrane receptors, IL-6Rα receptor and glycoprotein 130 (gp130). Binding of IL-6 to its receptors forms an active membrane protein complex and leads through gp130 to the activation of the Janus Kinase/Signal Transducer and Activation of Transcription (JAK/STAT) cascade and the Mitogen-Activated Protein Kinase (MAPK) cascade. IL-6 signalling is terminated by tyrosine phosphatases, Suppressor of Cytokine Signalling (SOCS) proteins and Protein Inhibitor of Activated STAT (PIAS) proteins. The balance between signalling pathways and suppressors of signalling mediates the final action of IL-6 in cells. Pneumon 2007; 20(1):154-167.