Tumor necrosis factor alpha (TNF-alpha) has been shown to be an important mediator of the lethal effects of endotoxin in several experimental models of septic shock. However, studies with a recombinant human interleukin-1 (IL-1) receptor antagonist protein (IL-1ra) suggest a role for IL-1 as a mediator of septic shock as well. In the present study, we show that mice treated in vivo with Corynebacterium parvum are primed for the production of interferon-gamma (IFN-gamma) and exhibit an enhanced capacity to produce serum IL-1 alpha, TNF-alpha, and IL-6 when challenged intravenously with lipopolysaccharide (LPS). The majority of C. parvum-treated mice die within 24 h of an LPS challenge. Pretreatment with a rat antimouse TNF-alpha monoclonal antibody (mAb) protected 90% of the animals against the lethal endotoxin challenge, while an anti-IFN-gamma mAb gave approximately 75% protection. The anti-IFN-gamma mAb also caused a reduction in LPS-induced serum TNF-alpha and IL-1 alpha. Anti-IL-1 alpha, anti-IL-1 beta, and anti-IL-6 neutralizing mAb did not protect against lethality when administered to mice prior to the LPS challenge. These results indicate that TNF-alpha and IFN-gamma are major mediators of endotoxin shock in C. parvum-treated mice. The results further suggest that the IFN-gamma produced by C. parvum-primed mice in response to an LPS challenge serves as a stimulus for enhanced production of TNF-alpha and IL-1 alpha. These findings are consistent with an increasing body of evidence suggesting a major role for IFN-gamma in lethal endotoxemia.