In most circumstances, NF-κB, which is essential for osteoclastogenesis, is activated following serine 32/36 phosphorylation of its cytosolic inhibitory protein, IκBα. In contrast to other cell types, IκBα, in bone marrow macrophages (BMMs), which are osteoclast precursors, is tyrosine-phosphorylated by c-Src kinase. To address the role of IκBα phosphorylation in osteoclastogenesis, we generated TAT fusion proteins containing wild-type IκBα (TAT-WT-IκB), IκBα lacking its NH2-terminal 45 amino acids (TAT-IκB46-317), and IκBα in which tyrosine residue 42, the c-Src target, is mutated into phenylalanine (TAT-Iκ B(Y42F)). TAT-IκB efficiently enters BMMs, and the NF-κB-inhibitory protein, once intracellular, is functional. While TAT-WT-IκB only slightly inhibits osteoclastogenesis, osteoclast recruitment is diminished >80% by TAT-IκB46-317, an event mirrored by dentin resorption. The fact that TAT alone does not impact osteoclastogenesis, which also resumes following withdrawal of TAT-IκB46-317, establishes that the mutant's anti-osteoclastogenic properties do not reflect toxicity. Affirming a functional role for IκB(Tyr42) in osteoclastogenesis, TAT-Iκ(Y42F) is as efficient as TAT-IκB 46-317 in blocking osteoclast differentiation. Thus, dominant-negative IκBα constructs block osteoclastogenesis, and Tyr42 is essential to the process, increasing the possibility that nonphosphorylatable forms of IκBα may be a means of preventing pathological bone loss.