The activator protein-1 (AP-1) transcription factor, comprising Jun and Fos family proteins, distinctly regulates various cellular processes, including those involved in inflammation. FOS like antigen 1 (Fra-1), a member of the Fos family, dimerizes with members of the Jun family and regulates gene expression in a context-dependent manner. Although respiratory toxicants are known to stimulate the expression of Fra-1 in the lung, whether Fra-1 promotes or decreases susceptibility to the development and progression of toxicant-induced lung diseasein vivois not well established. To determine the role of Fra-1 in LPS-induced acute lung injury and mortality, we administered LPS either intraperitoneally or intratracheally to Fra-1-sufficient (Fra-1+/+) and Fra-1-deficient (Fra-1Δ/Δ) mice. LPS-induced mortality, lung injury, inflammation, cytokine measurements, and AP-1 and NF-κB activities were then assessed in these mice.Fra-1Δ/Δ mice showed a greater resistance to LPS-induced mortality than did theirFra-1+/+counterparts. Consistent with this result, LPS-induced lung injury and inflammatory responses were markedly lower inFra-1Δ/Δ mice than inFra-1+/+ mice. Compared withFra-1+/+ mice,Fra-1Δ/Δ mice showed a reduced influx of neutrophils into the lungs, accompanied by a decreased expression of proinflammatory cytokines in response to treatment with LPS. The decreased inflammatory responses inFra-1Δ/Δ mice coincided with diminished and increased levels of NF-κB and c-Jun/AP-1 binding, respectively. These results demonstrate that Fra-1/AP-1 plays a key role in promoting LPS-induced injury and mortality in mice, and they suggest that targeting (i.e., inhibiting) this transcription factor may be a useful approach to dampening the adverse effects of exposure to endotoxins.