Recently, it has been found that differentiation into osteoclasts is induced by TNF-α. In this study, we investigated the effect of IL-12 on TNF-α-mediated osteoclastogenesis. When mouse bone marrow cells were cultured with TNF-α, osteoclast-like cells were formed. When they were cultured with both TNF-α and IL-12, the number of adherent cells in the bone marrow cells decreased in an IL-12 dose-dependent manner. A combination of IL-12 and TNF-α was necessary to induce death of the adherent cells in this culture system. Apoptotic alterations, which were indicated by morphological changes such as cellular atrophy, nuclear and cellular fragmentation, and biochemical changes such as DNA fragmentation, were observed in the adherent cells. Apoptosis of the adherent cells was markedly inhibited by anti-Fas ligand (FasL) Ab. RT-PCR and FACS analyses revealed that TNF-α up-regulated Fas transcription to lead to Fas expression on the surfaces of the adherent cells, whereas IL-12 could not induce Fas on the cells. In contrast, IL-12 induced FasL transcription to lead to FasL expression on the surfaces of nonadherent bone marrow cells, whereas TNF-α could not induce FasL on the cells. These results implied that apoptosis of the adherent cells in bone marrow cells might be caused by interaction between TNF-α-induced Fas on the adherent cells and IL-12-induced FasL on the nonadherent cells.