Interleukin 6 (IL-6) is a multifunctional 26 kD protein originally discovered in the medium of RNA-stimulated fibroblastoid cells. IL-6 appears to be directly involved in the responses that occur after infection and cellular injury, and it may prove to be important as IL-1 and TNF-a in regulating the acute phase response. IL-6 is reported to be produced by fibroblasts, activated T cells, activated monocytes or macrophages and endothelial cells. IL-6 acts upon a variety of cells including myeloid progenitor cells, T cells, B cells, and hepatocytes. In addition, IL-6 appears to interact with IL-2 in the proliferation of T lymphocytes. IL-6 potentiates the proliferative effect of IL-3 on multipotential hematopoietic progenitors. IL-6 plays a critical role in B-cell differentiation to plasma cells and is a potent growth factor for plasmacytoma and myeloma. IL-6 is a very useful culture supplement for the generation of a high number of antibody-producing hybridomas. Primarily produced at sites of acute and chronic inflammation, IL-6 is secreted into the serum and induces a transcriptional inflammatory response through interleukin 6 receptor, alpha. The functioning of IL-6 is implicated in a wide variety of inflammation-associated disease states including diabetes mellitus and systemic juvenile rheumatoid arthritis.