Several studies indicate that IFN-γ facilitates systemic inflammation during endotoxin-induced shock. However, the pathobiology of IFN-γ in clinically relevant models of septic shock, such as CLP, is not well understood. In this study, the role of IFN-γ in the pathogenesis of CLP-induced septic shock was evaluated by examining IFN-γ production at the tissue and cellular levels. The impact of IFN-γ neutralization on systemic inflammation, bacterial clearance, and survival was also determined. Following CLP, concentrations of IFN-γ in plasma and peritoneal lavage fluid were low in comparison with concentrations of IL-6 and MIP-2, as was IFN-γ mRNA expression in liver and spleen. The overall percentage of IFN-γ+ splenocytes was <5% after CLP and not statistically different from control mice. Intracellular IFN-γ was present in a large proportion of peritoneal exudate cells after CLP, primarily in infiltrating myeloid cells and NK cells. i.p. myeloid cell activation was decreased in IFN-γKO mice, and plasma concentrations of IL-6 and MIP-2 were significantly lower in IFN-γKO mice and in mice treated with anti-IFN-γ compared with controls, but bacterial clearance was not affected. IFN-γKO mice were resistant to CLP-induced mortality when treated with systemic antibiotics. However, neutralization of IFN-γ with blocking antibodies did not improve survival significantly. These studies show that IFN-γ facilitates the proinflammatory response during CLP-induced septic shock. However, neutralization of IFN-γ did not improve survival uniformly.