Osteoprotegerin (OPG;) and osteoclast differentiation factor (ODF) are crucial regulators of osteoclastogenesis, To determine the biological role of interleukin (IL)-18 produced by stromal/osteoblaastc cells in osteoclastogenesis, we examined the effects of IL-18 on the OPG and ODF mRNA levels in these cells. When bone marrow stromal ST2 cells, osteoblastic MC3T3-E1 cells, and mouse calvarial osteoblasts were stimulated with IL-18, the expression of OPG mRNA, but not ODF mRNA, was transiently increased, its expression reaching a maximal level at 3 h after the beginning of the culture. In accordance with this observation, all these cells expressed the mRNAs of two IL-18 receptor components and MyD88, an adapter molecule involved in IL-18 signaling. Moreover, in these cells, mitogen-activated protein kinase was phosphorylated after stimulation with IL-18. These results suggest that stromal/osteoblastic cells are IL-ls-responsive cells and that IL-18 may inhibit osteoclastogenesis by upregulating OPG expression, without stimulation of ODF production, in stromal/osteoblastic cells.