The amino acid sequence of IL-6 was actually first reported as a so-called 26 к Da protein by Content et al. in 1982. This product of fibroblasts was thought to have antiviral activity and was therefore mis-identified as IFN-B2. A protein termed В cell differentiation factor (BCDF) or В cell-stimulating factor 2 (BSF-2) based on its capacity to induce antibody secretion by В cells was cloned by Hirano et al in 1986. BCDF/BSF-2 was renamed IL-6 and found to be identical to IFN-B2 (Zilberstein et al.s 1986; Poupart el al., 1987). Although IL-6 does not have any antiviral activity, it may induce cells to produce IFN. IL-6 also was identified as an 'hepatocyte-stimulating factor' based on its capacity to induce acute phase protein production by cultured hepatocytes (Andus et al., 1988). IL-6 also plays an important role in early hematopoietic cell proliferation and differentiation. A number of ligands that are closely related to IL-6 in using a common receptor chain were subsequently discovered as will be discussed. It is beyond the scope of this chapter to discuss the discovery of all the remaining interleukins in detail, but we will discuss the discovery of some of the receptors with which the interleukins interact.