W hile TNF-α is pivotal to the pathogenesis of inflammatory osteolysis, the means by which it recruits osteoclasts and promotes bone destruction are unknown. We find that a pure population of murine osteoclast precursors fails to undergo osteoclastogenesis when treated with TNF-α alone. In contrast, the cytokine dramatically stimulates differentiation in macrophages primed by less than one percent of the amount of RANKL (ligand for the receptor activator of NF-κB) required to induce osteoclast formation. Mirroring their synergistic effects on osteoclast differentiation, TNF-α and RANKL markedly potentiate NF-κB and stress-activated protein kinase/c-Jun NH₂-terminal kinase activity, two signaling pathways essential for osteoclastogenesis. In vivo administration of TNF-α prompts robust osteoclast formation in chimeric animals in which β-galactosidase positive, TNF-responsive macrophages develop within a TNF-nonresponsive stromal environment. Thus, while TNF-α alone does not induce osteoclastogenesis, it does so both in vitro and in vivo by directly targeting macrophages within a stromal environment that expresses permissive levels of RANKL. Given the minuscule amount of RANKL sufficient to synergize with TNF-α to promote osteoclastogenesis, TNF-α appears to be a more convenient target in arresting inflammatory osteolysis.