Interleukin‐6 (IL‐6) is a multifunctional cytokine that plays a central role in host defense due to its wide range of immune and hematopoietic activities and its potent ability to induce the acute phase response. Overexpression of IL‐6 has been implicated in the pathology of a number of diseases including multiple myeloma, rheumatoid arthritis, Castleman's disease, psoriasis, and post‐menopausal osteoporosis. Hence, selective antagonists of IL‐6 action may offer therapeutic benefits. IL‐6 is a member of the family of cytokines that includes interleukin‐11, leukemia inhibitory factor, oncostatin M, cardiotrophin‐1, and ciliary neurotrophic factor. Like the other members of this family, IL‐6 induces growth or differentiation via a receptor‐system that involves a specific receptor and the use of a shared signaling subunit, gp130. Identification of the regions of IL‐6 that are involved in the interactions with the IL‐6 receptor and gp130 is an important first step in the rational manipulation of the effects of this cytokine for therapeutic benefit. In this review, we focus on the sites on IL‐6 which interact with its low‐affinity specific receptor, the IL‐6 receptor, and the high‐affinity converter gp130. A tentative model for the IL‐6 hexameric receptor ligand complex is presented and discussed with respect to the mechanism of action of the other members of the IL‐6 family of cytokines.