Interleukin‐10 (IL‐10) is a potent inhibitor of lipopolysaccharide (LPS)‐induced tumor necrosis factor (TNF) production and has been shown to protect mice from endotoxin shock. As IFN‐γ is another important mediator of LPS toxicity, we studied the effects of IL‐10 on LPS‐induced IFN‐γ synthesis in vitro and in vivo. First, we found that the addition of recombinant human IL‐10 (rhIL‐10) (10 U/ml) to human whole blood markedly suppressed LPS‐induced IFN‐γ release while neutralization of endogenously synthesized IL‐10 resulted in increased IFN‐γ levels. The ability of rIL‐10 to inhibit LPS‐induced IFN‐γ synthesis was also observed in vivo in mice. Indeed, administration of 1000 U recombinant mouse IL‐10 (rmIL‐10) 30 min before and 3 h after challenge of BALB/c mice with 100 μg LPS resulted in a threefold decrease in peak IFN‐γ serum levels. We then examined the production and the role of IL‐10 during murine endotoxemia. We found that LPS injection causes the rapid release of IL‐10, peak IL‐10 serum levels being observed 90 min after LPS challenge. Neutralization of endogenously produced IL‐10 by administration of 2 mg JES5‐2A5 anti‐IL‐10 monoclonal antibody (mAb) 2h before LPS challenge resulted in a marked increase in both TNF and IFN‐γ serum levels while irrelevant isotype‐matched mAb had no effect. The enhanced production of inflammatory cytokines in anti‐IL‐10 mAb‐treated mice was associated with a 60% lethality after injection of 500 μg LPS, while all mice pretreated with control mAb survived. We conclude that the rapid release of IL‐10 during endotoxemia is a natural antiinflammatory response controlling cytokine production and LPS toxicity.