Interleukin 6 (IL-6) is a multifunctional protein that plays important roles in host defense, acute phase reactions, immune responses, and hematopoiesis.(1) Synonyms for IL-6 include: B cell stimulatory factor-2 (BSF-2),(2) hybridoma/plasmacytoma growth factor,(3) hepatocyte stimulating factor, cytotoxic T cell differentiation factor and macrophage-granulocyte inducing factor 2A (MGI-2A).(1) IL-6 is expressed by a variety of normal and transformed cells including T cells, B cells, monocytes/macrophages, fibroblasts, hepatocytes, keratinocytes, astrocytes, vascular endothelial cells, mesangial cells, osteoblasts, carcinomas, sarcomas, myelomas, glioblastomas, and melanomas. The production of IL-6 is up-regulated by numerous signals including mitogenic or antigenic stimulation, lipopolysaccharide, calcium ionophore, IL-1, IL-2, IFN, TNF, PDGF, and viruses. IL-4 and IL-13 inhibit IL-6 expression in monocytes. (4-6) Natural human and murine IL-6 are glycoproteins containing N-and/or O-linked carbohydrates (human IL-6 contains two potential N-glycosylation sites, while mouse IL-6 has none). In comparison with mouse IL-6, human IL-6 exhibits approximately 65% sequence homology at the nucleotide level, and 42% homology at the amino acid level. Although human and mouse IL-6 are equally active on mouse cells, mouse IL-6 is not active on human cells. IL-6 is predicted to have a four helix-bundle type tertiary structure found in a number of other cytokines including growth hormone, EPO, G-CSF, OSM, IL-11, CNTF, LIF, MGF, Prolactin, etc. (7-9) The gene structures of these cytokines also show varying degrees of relatedness. Based on these criteria, it has been suggested that these cytokines evolved from a common ancestral gene. Results of structure-function studies of IL-6 and other four a-helix bundle cytokines indicated that the c-terminal (helix D) regions of these cytokines are primarily responsible for binding to the receptors.(1. 4) Interleukin 6 exerts multiple functions on numerous target cells. IL-6 plays an important role in immune functions. It has effects on B cell differentiation and antibody production, on cytotoxic T cell differentiation, on T cell activation, growth and differentiation, and on the induction of IL-2Ra chain expression and IL-2 production in T cells.(11-13) In hemopoiesis, IL-6 has blast cell growth factor activity and can synergize with IL-3 to shorten the G0 period of early hemopoietic progenitors.(14) In addition, IL-6 has been found to synergize with IL-3 in megakaryocyte development, increasing platelet numbers in vivo and the number, size and average ploidy value of megakaryocyte colonies formed from mouse or human bone marrow cells in vitro.(15-18) Similarly to IL-11 and LIF, IL-6 can induce the synthesis of hepatic acute phase proteins both in vivo and in vitro.(19) IL-6 has growth factor activities and will stimulate the growth of hybridomas, plasmacytomas, myelomas, sarcomas,(20, 21) carcinomas,(22) EBV-transformed B cells,(23) keratinocytes, and mesangial cells. In contrast to its growth stimulatory activities, IL-6 is also a growth inhibitor for a number of leukemia and carcinoma cell lines. Additional bioactivities attributed to IL-6 include: induction of terminal differentiation of M1 myeloid leukemic cells;(24) the differentiation and survival of neuronal cells;(25, 26) and the activation of osteoclast development.(10) Although IL-6 was also discovered as an antiviral factor produced by human diploid fibroblasts, the question of whether or not IL-6 has antiviral activity is controversial. Many groups have been consistently unable to find any antiviral activity for recombinant human IL-6.(1) The various activities of IL-6 described above suggest that this factor plays a major role in the mediation of the inflammatory and immune responses. This IL-6 ELISA is a 3.5 hour solid phase immunoassay readily applicable to measure IL-6 levels in cell culture supernatant, serum, plasma, and other biological fluids in the range of 0 to 400 pg/mL. It showed no cross reactivity with various other cytokines superfamily proteins.