The ability of monophosphoryl lipid A (MLA) to provide prophylactic protection against septic shock was evaluated in a mouse model of induced endotoxin hypersensitivity. Treatments of hypersensitized animals with low doses of MLA attenuated endotoxin lethality and endotoxin-mediated liver damage. These effects were related to the ability of MLA to suppress accumulation of TNF-alpha and IFN-gamma in the bloodstream of animals. MLA treatments had only a modest effect in suppressing the accumulation of nitrate in the bloodstream. This implied that MLA did not suppress induction of macrophage and hepatocyte nitric oxide synthetases that contribute to antimicrobial defense and protect against endotoxin-mediated liver damage. The MLA treatments did not appear to compromise inflammatory defenses against local infection since locally recruited leukocytes remained responsive to endotoxin after hypersensitivity had been attenuated. In agreement with these findings, other studies have shown that the induction of endotoxin tolerance by MLA parallels the induction of resistance of animals to lethal challenges with either Gram negative or Gram positive bacteria. As predicted from preclinical studies, human trials of the clinical form of MLA (MPL-immunostimulant) have confirmed that MLA could attenuate systemic responses to endotoxin in normal volunteers, including the attenuation of blood cytokine accumulation and attenuation of symptomatic responses.