It has been reported that TNF-alpha plays an important role in bone resorption in pathological conditions. IL-12, which is a T cell mediator, is also an important inflammatory cytokine. We previously reported that IL-12 induces apoptosis in bone marrow cells treated with TNF-alpha in vitro via an interaction between TNF-alpha-induced Fas and IL-12-induced Fas ligand (FasL), and that, as a result, osteoclastogenesis is inhibited. The purpose of this study was to investigate the effects of IL-12 on TNF-alpha-mediated osteoclastogenesis in vivo. We administered TNF-alpha with and without IL-12 into the supracalvaria in mice. The numbers of osteoclasts in the sutures in the calvaria were higher in mice administered TNF-alpha than in control mice not administered TNF-alpha. The numbers of osteoclasts in mice administered both TNF-alpha and IL-12 were lower than those in mice administered only TNF-alpha. Next, we determined the levels of mRNAs for cathepsin K and tartrate-resistant acid phosphatase (TRAP). mRNA levels were increased in mice administered TNF-alpha compared with control mice, but not in mice administered both TNF-alpha and IL-12. We also evaluated the amounts of tartrate-resistant acid phosphatase 5b (TRACP 5b) in mouse sera. The levels of TRACP 5b in mice administered TNF-alpha were higher than those in control mice. On the other hand, in mice administered both TNF-alpha and IL-12, the levels were lower than those in mice administered TNF-alpha alone. Fas and FasL expression levels were analyzed by real-time RT-PCR. The levels of Fas mRNA were increased in the calvaria of mice administered TNF-alpha compared with control mice, while those of FasL mRNAs were increased in the calvaria of mice administered IL-12. In TdT-mediated dUTP-biotin nick end-labeling (TUNEL) assays, many apoptotic cells were found in the sutures in the calvaria of mice administered both TNF-alpha and IL-12. IL-12 also inhibited TNF-alpha-induced osteoclastogenesis in mice whose T cells were blocked by anti-CD4 and anti-CD8 antibodies. These results suggest that IL-12 inhibits TNF-alpha-mediated osteoclastogenesis and induces apoptotic changes through an interaction between TNF-alpha-induced Fas and IL-12-induced FasL, in vivo, via a T cell-independent mechanism.