Endotoxin has been identified as a principal mediator of sepsis, often with resulting multiple organ failure. Although interferon γ (IFN-γ) has a central role in controlling bacterial infection through the activation of macrophages and T lymphocytes, it can also enhance the harmful effects of the inflammatory response. To examine the role of IFN-γ in lipopolysaccharide (LPS)-induced injury, we administered LPS (20 or 800 μg/mouse) alone or as low-dose LPS (20 μ/mouse) 7 days after heat-killed Propionibacterium acnes (P. acnes) injection into wild-type C57BL/6 (B6) mice or IFN-γ-deficient (GKO) mice (B6 background). Although low-dose (20 μg) LPS alone had no effect on survival, the administration of 800 μg LPS alone resulted in 100% mortality in both B6 and GKO mice without significant hepatic mononuclear cellular infiltration or differences in elevated plasma tumor necrosis factor α (TNF-α), interleukin 6 (IL-6), and IL-12 levels. In contrast, mortality after low-dose (20 μg) LPS challenge in P. acnes-primed B6 mice was 100%, but 0% in GKO mice. In vivo plasma cytokine (IFN-γ, TNF-α, IL-6, and IL-12) levels and in vitro cytokine production by hepatic mononuclear cells were significantly higher in B6 mice compared with GKO mice. Associated hepatic mononuclear cellular infiltration, multifocal liver necrosis, hepatomegaly, and splenomegaly were found in B6 mice, but not in GKO mice. Finally, the anti-inflammatory NK1.1+CD4+ cell proportion of hepatic infiltrating mononuclear cell numbers 7 days after P. acnes administration was significantly reduced in B6 compared with GKO mice, whereas the proportion of inflammatory NK1.1+CD4- cells was increased. In conclusion, these data suggest that IFN-γ mediates P. acnes-primed low-dose LPS injury through the hepatic infiltration of mononuclear cells and the subsequent elevation of inflammatory cytokines after LPS challenge, whereas the lethal effects of high-dose LPS alone does not depend on the presence of IFN-γ.