Despite the differences in the molecular structure between lipopolysaccharides (LPS) isolated from Escherichia coli, Klebsiella pneumoniae or Salmonella typhimurium, the potential differences in their biological effects in vivo have not been investigated. In the present study, TNF and LT double knock-out (TNF-/-LT-/-) mice were almost as susceptible as TNF+/+LT+/+ controls to S. typhimurium LPS, but they were significantly more resistant to lethal endotoxemia induced by E. coli or K. pneumoniae LPS. The effect was not due to endotoxin-associated proteins. In the knock-out mice, this difference in lethality was accompanied by decreased interleukin-1 (IL-1) and interferon-gamma (IFN-gamma) production after challenge with E. coli LPS, whereas after S. typhimurium LPS more IL-1 and IFN-gamma were produced. In contrast, more IL-10 was produced after challenge of mice with E. coli LPS than with S. typhimurium LPS. The hypothesis that a combination of pro-inflammatory cytokines is responsible for the mortality after S. typhimurium LPS was suggested by experiments in mice deficient in IL-1beta-converting enzyme (ICE-/- mice). ICE-/-mice, lacking mature IL-1beta and IL-18, but also defective in IFN-gamma and TNF production, were completely protected against both E. coli and S. typhimurium LPS. Experiments in Toll-like receptor (TLR)-4 defective mice suggested that the difference is not due to differential activation of TLR4. In conclusion, TNF and LT play a central role in the lethality due to E. coli LPS, whereas the lethal effects of S. typhimurium LPS are mediated through mechanisms also involving other cytokines such as IFN-gamma, IL-1 and IL-18.