Interleukin 6 receptor is a protein complex consisting of an IL-6 receptor subunit (IL6 Rα) and interleukin 6 signal transducer Glycoprotein 130 (gp130). A soluble form of the IL-6 Rα with a molecular weight of approximately 50 kDa apparently arises from proteolytic cleavage of membrane-bound IL-6 Rα. The receptor has been shown to bind to IL-6 in solution and to augment the activity of IL-6 as a result of the binding of the IL-6/IL-6 sRα complex to membrane-bound gp130. The regulation in vivo of the shedding of the soluble IL-6 R and the function and significance of these soluble receptors in biological fluids is not currently understood. It has been suggested, however, that pathological states involving elevated levels of IL-6 might also be associated with increased production of soluble IL-6 R.