Interleukin-6 (IL-6) is a pro-inflammatory cytokine that also has an important role in immunity. Mouse IL-6 appears to be directly involved in the responses that occur after infection and injury and may prove to be as important as IL-1 in regulating the acute phase response. Mouse IL-6 is reported to be produced 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. IL-6 has a wide variety of biological functions: it plays an essential role in the final differentiation of B-cells into Ig-secreting cells, it induces myeloma and plasmacytoma growth, nerve cells differentiation in hepatocytes, and acute phase reactants.