Mouse interleukin-6 (IL-6) is a 24 kDa protein composed of 184 amino acids and originally discovered in the medium of lymphocyte cell culture stimulated by fibroblastoid cells. Upregulation by IL-1, PDGF, IFN β, TNF-α was found in several cell types and downregulation by glucocorticoids had been reported recently. IL-4, TGFβ. IL-6 appears to be directly involved in the responses that occur after infection and cellular injury. IL-6 may prove to be as important as IL-β and TNFα in regulating the acute phase response. IL-6 has also been implicated in regulating adipose mass. IL-6 is generated by fibroblasts, activated T cells, activated monocytes or macrophages and endothelial cells. It acts upon a variety of cells including fibroblasts, 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.