Interleukin-1 (IL-1) designates two proteins, IL-1α and IL-1β, which are the products of distinct genes, but recognize the same cell surface receptors. IL-1α and IL-1β are structurally related polypeptides that show approximately 25% homology at the amino acid level. Both proteins are produced by a wide variety of cells in response to stimuli such as those produced by inflammatory agents, infections, or microbial endotoxins. The proteins are synthesized as 31 kDa precursors that are subsequently cleaved into proteins with molecular weights of approximately 17.5 kDa. The specific protease responsible for the processing of IL-1β, designated interleukin 1β-converting enzyme (ICE), has been described. Mature human and mouse IL-1β share approximately 75% amino acid sequence identity and human IL-1β has been found to be active on murine cell lines.