Interleukin-18 (IL-18), previously known as interferon-gamma (IFN-γ)-inducing factor, is an activator of NK cells and a co-inducer of Th1 cytokines. IL-18 is produced from Kupffer cells, activated macrophages, keratinocytes, intestinal epithelial cells, osteoblasts, adrenal cortex cells and murine diencephalon. IL-18 was expressed in E.coli and purifed by conventional chromatography, after refolding of the isolated inclusion bodies in a renaturation buffer.