We explored the role of the transcription factor c-Fos in lipopolysaccharide (LPS)-induced cytokine response using mice lacking c-Fos (Fos−/− mice). Compared with wild-type controls, Fos−/− macrophages and mice showed significantly enhanced production of tumour necrosis factor (TNF)-α, interleukin (IL)-6 and IL-12 p40, but reduced production of the anti-inflammatory cytokine IL-10. Bandshift analysis revealed that LPS-induced NF-κB binding activity to a functional site in the TNF-α promoter was significantly higher in Fos−/− than in wild-type macrophages. Using telemetry, we monitored body temperature and heart rate after LPS injection and found that Fos−/− mice undergo more severe hypothermia and bradycardia than wild-type mice. Such shock responses in Fos−/− mice were significantly reversed by neutralizing TNF-α. These data reveal a novel in vivo role for c-Fos as an anti-inflammatory transcription factor acting through suppression of NF-κB activity.