We describe the gene structure, regulation, signal transduction. and functions of a cytokine, interleukin (IL)-32. An IL-18 unresponsive cell was converted to a responsive cell by transfection of the IL-18 receptor β chain, and IL-18-induced microarray revealed high expression of a cytokine-like gene. Although IL-32 does not share sequence homology with known cytokine families, IL-32 induces various cytokines, human TNFα, and IL-8 in THP-1 monocytic cells as well as mouse TNFα and MIP-2 in Raw macrophage cells. IL-32 activates typical cytokine signal pathways of nuclear factor-kappa B (NF-κB) and p38 mitogen-activated protein kinase. IL-32 mRNA is highly expressed in immune tissue rather than other tissues. Human IL-32 exists as four splice variants, and IL-32 from other species were found as expressed sequence tag clones in the databank. Induced in human peripheral lymphocyte cells after mitogen stimulation, in human epithelial cells by IFNγ, and in NK cells after exposure to the combination of IL-12 plus IL-18, IL-32 may play a role in inflammatory/autoimmune diseases.