Interleukin‐18 binding protein is a novel glycoprotein that we successfully cloned and expressed. First, murine interleukin‐18 binding protein was purified from the sera of mice with endotoxin shock using ligand affinity chromatography. The murine interleukin‐18 binding protein cDNA was cloned after RT‐PCR using mixed primer pair sequences based on partial murine interleukin‐18 binding protein amino acid sequence analysis. Subsequently, human interleukin‐18 binding protein cDNA was cloned from cDNA libraries of normal human liver using murine interleukin‐18 binding protein cDNA as a probe. Next, we transiently expressed recombinant human and murine interleukin‐18 binding proteins in COS‐1 cells and purified them from culture supernatants. Both recombinant interleukin‐18 binding proteins did not exhibit species specificity and prevented interleukin‐18 binding to its receptor. In addition, they inhibited interleukine‐18 dependent IFN‐γ production from KG‐1 cells effectively. These results suggest that the interleukin‐18 binding protein may possess interleukine‐18 antagonist activity.