After a sublethal exposure to lipopolysaccharide (LPS) or to lipid A, which is that portion of the LPS molecule associated with endotoxicity, a transient period ensues during which a normally responsive individual is rendered hyporesponsive to LPS-induced toxicity. This period has been defined as early-phase endotoxin tolerance. Recently, a nontoxic derivative of lipid A from Salmonella typhimurium, monophosphoryl lipid A (MPL), was isolated and purified. In this study, we assessed the ability of MPL to induce early endotoxin tolerance. Initial injection of MPL resulted in a dose-dependent stimulation of both serum colony-stimulating factor and serum interferon, indicators of in vivo LPS responsiveness. In contrast, MPL failed to induce the symptoms of endotoxicity which are normally seen after injection of even sublethal amounts of intact endotoxin or lipid A preparations. Injection of MPL on day 0 reduced significantly the amount of LPS-induced serum colony-stimulating factor and interferon produced upon challenge with Escherichia coli LPS 3 days later and also mitigated toxic manifestations, as evidenced by a marked increase in the 50% lethal dose. Like the early tolerance induced by wild-type (toxic) LPS, MPL-induced tolerance was characterized by an accompanying elevation in the number of bone marrow-derived macrophage progenitor cells and by an alteration in bone marrow cell sizing profiles. These results indicate that MPL is effective in inducing a state of LPS-hyporesponsiveness without the toxic side effects of endotoxin and that the structural component(s) necessary for induction of early-phase endotoxin tolerance is contained within MPL.