Abstract Mice lacking Tbk1 activity exhibit immune cell infiltrates in multiple tissues, altered circulating immune cell compartments, and increased susceptibility to LPS-induced lethality. 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 Tbk1Δ allele encodes a truncated Tbk1Δ protein that is catalytically inactive and expressed at very low levels. Upon LPS stimulation, macrophages from Tbk1Δ/Δ mice produce normal levels of proinflammatory cytokines (e.g., TNF-α), but IFN-β and RANTES expression and IRF3 DNA-binding activity are ablated. Three-month-old Tbk1Δ/Δ 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+/Δ littermates. Skin from 2-week-old Tbk1Δ/Δ mice is characterized by reactive changes, including hyperkeratosis, hyperplasia, necrosis, inflammatory cell infiltrates, and edema. In response to LPS challenge, 3-month-old Tbk1Δ/Δ mice die more quickly and in greater numbers than their Tbk1+/+ and Tbk1+/Δ counterparts. This lethality is accompanied by an overproduction of several proinflammatory cytokines in the serum of Tbk1Δ/Δ mice, including TNF-α, GM-CSF, IL-6, and KC. This overproduction of serum cytokines in Tbk1Δ/Δ 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.