TBK1 is critical for immunity against microbial pathogens that activate TLR4- and TLR3-dependent signaling pathways. To address the role of TBK1 in inflammation, mice were generated that harbor two copies of a mutant Tbk1 allele. This Tbk1(Delta) allele encodes a truncated Tbk1(Delta) protein that is catalytically inactive and expressed at very low levels. Upon LPS stimulation, macrophages from Tbk1(Delta/Delta) mice produce normal levels of proinflammatory cytokines (e.g., TNF-alpha), but IFN-beta and RANTES expression and IRF3 DNA-binding activity are ablated. Three-month-old Tbk1(Delta/Delta) mice exhibit mononuclear and granulomatous cell infiltrates in multiple organs and inflammatory cell infiltrates in their skin, and they harbor a 2-fold greater amount of circulating monocytes than their Tbk1(+/+) and Tbk1(+/Delta) littermates. Skin from 2-week-old Tbk1(Delta/Delta) mice is characterized by reactive changes, including hyperkeratosis, hyperplasia, necrosis, inflammatory cell infiltrates, and edema. In response to LPS challenge, 3-month-old Tbk1(Delta/Delta) mice die more quickly and in greater numbers than their Tbk1(+/+) and Tbk1(+/Delta) counterparts. This lethality is accompanied by an overproduction of several proinflammatory cytokines in the serum of Tbk1(Delta/Delta) mice, including TNF-alpha, GM-CSF, IL-6, and KC. This overproduction of serum cytokines in Tbk1(Delta/Delta) mice following LPS challenge and their increased susceptibility to LPS-induced lethality may result from the reactions of their larger circulating monocyte compartment and their greater numbers of extravasated immune cells.