IL-1β is a potent modulator of immune and inflammatory responses. Murine IL-1β is initially synthesized as an inactive 33-kDa pro-molecule that is activated by proteolytic cleavage between Asp-117 and Val-118 to generate the 17-kDa mature IL-1β protein. This cleavage is catalyzed by a specific protease that has been designated the IL-1β converting enzyme (or IL-1β convertase). We have used a human IL-1β convertase cDNA to isolate murine convertase cDNA from a WEHI-3 library. These cDNA predicted that the murine convertase is a 402-residue protein. Overall, the murine convertase showed 71% nucleotide and 62% predicted amino acid sequence identity with the human convertase. Southern blot analysis of interspecific backcross mice indicated that the murine IL-1β convertase is encoded by a single copy gene located on murine chromosome 9. The murine convertase showed broad constitutive expression, being detected in mononuclear phagocyte and T lymphocyte cell lines as well as in spleen, heart, brain, and adrenal glands. The expression of the murine convertase in mononuclear phagocytes was up-regulated by treatment with LPS or rIFN-γ. These studies establish that the IL-1β convertase is an evolutionarily conserved, widely expressed enzyme that can be regulated at a pretranslational level.