IL-18 (IFN-gamma-inducing factor, IGIF) is a novel proinflammatory cytokine that aμgments natural killer (NK) activity in spleen cells. It also has other biological properties such as activation of NF-kB, stimulation of Fas ligand expression, induction of CC and CXC chemokines. IL-18 increases production of competent HIV. IL-18 is related to the IL-1 family. One of the IL-18 receptor chains is the IL-1 receptor-related protein, a member of the IL-1R family. Like IL-1β, IL-18 is first synthesized as a leaderless precursor requiring the IL-1β converting enzyme (ICE) for cleavage into an active molecule. IL-18 may play a role in tissue injury in inflammatory reactions.