Macrophage tumour necrosis factor-alpha (TNF-α) production is thought to represent an important pathogenic mechanism by which Gram-negative sepsis is mediated. We compared the effects of caecal ligation and puncture (CLP) on endotoxin-sensitive (C3H HeSnJ) and endotoxin-resistant (C3H HeJ) mice. Mortality after CLP for C3H HeSnJ mice compared with C3H HeJ mice was not significantly different (32% and 55%, respectively). When survivors were injected with lipopolysaccharide intraperitoneally on the 7th day after CLP, the mortality rate was 82% for C3H HeSnJ mice versus 0% for C3H HeJ mice (P< 0.0001). Serum endotoxin levels at 24 h after CLP were only slightly elevated. Serum TNF levels and peritoneal macrophage TNF production were undetectable in C3H HeJ mice and were only slightly elevated in C3H HeSnJ mice by 24 h after CLP. Peritoneal macrophage mRNA levels for TNF-α, IL-1β, and I-Aα displayed a similar pattern in the two strains of mice, with a 2-to 3-fold increase in TNF-α and IL-1β mRNA levels by 24 h and a sharp decrease in I-Aα mRNA by 24 h. The cause of mortality in mice that undergo CLP cannot be attributed to overwhelming endotoxemia and/or TNF production.