We have established by differential display polymerase chain reaction of mRNA that interleukin (IL)-18 is expressed by osteoblastic stromal cells. The stromal cell populations used for comparison differed in their ability to promote osteoclast-like multinucleated cell (OCL) formation. mRNA for IL-18 was found to be expressed in greater abundance in lines that were unable to support OCL formation than in supportive cells. Recombinant IL-18 was found to inhibit OCL formation in cocultures of osteoblasts and hemopoietic cells of spleen or bone marrow origin. IL-18 inhibited OCL formation in the presence of osteoclastogenic agents including 1α,25-dihydroxyvitamin D₃, prostaglandin E₂, parathyroid hormone, IL-1, and IL-11. The inhibitory effect of IL-18 was limited to the early phase of the cocultures, which coincides with proliferation of hemopoietic precursors. IL-18 has been reported to induce interferon-γ (IFN-γ) and granulocyte/macrophage colony-stimulating factor (GM–CSF) production in T cells, and both agents also inhibit OCL formation in vitro. Neutralizing antibodies to GM–CSF were able to rescue IL-18 inhibition of OCL formation, whereas neutralizing antibodies to IFN-γ did not. In cocultures with osteoblasts and spleen cells from IFN-γ receptor type II–deficient mice, IL-18 was found to inhibit OCL formation, indicating that IL-18 acted independently of IFN-γ production: IFN-γ had no effect in these cocultures. Additionally, in cocultures in which spleen cells were derived from receptor-deficient mice and osteoblasts were from wild-type mice and vice versa, we identified that the target cells for IFN-γ inhibition of OCL formation were the hemopoietic cells. The work provides evidence that IL-18 is expressed by osteoblasts and inhibits OCL formation via GM–CSF production and not via IFN-γ production.