Interleukin-1 (IL-1) is a mediator of endotoxin shock and IL-1 receptor blockade has been shown to have therapeutic efficacy against endotoxic shock and sepsis in laboratory models. The current studies were designed to characterize the efficacy of a murine monoclonal IL-1 receptor antibody (IL-1rab) against endotoxin (LPS) lethality and to investigate whether combined anticytokine therapy using the IL-1rab and a highly specific polyclonal rabbit anti-mouse TNF antibody (TNF Ab) could provide additive or synergistic efficacy against LPS lethality in C57B1/6 female mice. A single intraperitoneal (ip) dose of IL-1rab, 0.1 or 0.2 mg, significantly reduced lethality from LPS, 30 to 40 mg/kg ip, compared to nonimmune IgG, 0.1 or 0.2 mg, in control mice (P₂ < 0.05). Treatment with IL-1rab was effective when administered from 6 hr before to 1 hr after LPS. After LPS, circulating levels of IL-6 were significantly lower in IL-1rab-treated mice [IL-6 (ng/ml) 2 h after LPS: IgG, 100 ± 25, IL-1rab, 41 ± 8; 4 h after LPS: IgG, 46 ± 13, IL-1rab, 8 ± 1; P₂ < 0.05 and 0.03, respectively]. Northern blot analysis showed that IL-1rab markedly lowered IL-6 gene expression after LPS. Combined treatment with IL-1rab and TNF Ab did not result in any improvement in survival after LPS compared to either agent alone. These results indicate that an IL-1 receptor antibody has therapeutic efficacy against LPS and significantly decreases IL-6 production. Combined anticytokine treatment (against IL-1 and TNF) of endotoxic or septic shock does not appear justified on the basis of these results.