We genetically engineered human myelomonocytic KG-I cells by introducing cDNA of murine interleukin-18 receptor (MuIL-18R) and established human cells which were capable of responding to MuIL-18. These cells expressed larger number of MuIL-18R (> 13,000 sites/cell) than intrinsic human IL-18 receptor (HuIL-18R) (< 2,500 sites/cell). And the cells responded to MuIL-18 as well as to HuIL-18 in a dose-dependent manner, and produced large amounts of interferon-gamma (IFN-gamma). We could estimate the amount of murine IL-18 based on the amounts of IFN-gamma produced by these cells. The stoichiometry was observed up to 150 ng/ml of MuIL-18. By using these cells, a large amount of MuIL-18 (448 +/- 89.2 ng/ml) was detected in sera of Propionibacterium acnes (P. acnes)/lipopolysaccharide (LPS)-treated endotoxic mice (the same conditions in which IL-18 was first identified). These cells provide us with a useful tool for determining the bioactivity of MuIL-18.