When incubated with lipopolysaccharide (LPS) in the presence of plasma, neutrophils become primed for enhanced release of superoxide in response to triggering by formyl-Met-Leu-Phe (fMLP). The effect of LPS on phagocytes is inhibited by a synthetic lipid A precursor, LA-14-PP (lipid IVa) or by LPS from Rhodobacter sphaeroides (Rs). We studied the mechanisms by which LA-14-PP or Rs-LPS inhibited LPS-induced responses. When neutrophils were exposed to LA-14-PP or Rs-LPS for 3 min and then to Escherichia coli-LPS, the antagonists inhibited priming for superoxide release, and also blocked up-regulation of CD11b and adherence. This inhibition was dependent on plasma, was not overcome by higher amounts of E. coli-LPS or plasma, and was not observed at 0 degrees C, suggesting that E. coli-LPS was not able to interact with its receptor or other cellular recognition molecule in neutrophils that had been exposed to the antagonists. The alternative possibility that LA-14-PP or Rs-LPS depleted a plasma cofactor, resulting in inhibition of priming, was investigated by using LPS from Porphyromonas gingivalis (Pg) and Bordetella pertussis (Bp). These LPS primed neutrophils in a plasma-dependent and CD14-dependent manner, but were not blocked by LA-14-PP or Rs-LPS. When sub-optimal concentrations of plasma were exposed to LA-14-PP or Rs-LPS, and then mixed with Pg-LPS or Bp-LPS, followed by incubation with neutrophils, priming and up-regulation of CD11b were inhibited, and this inhibition was overcome by increasing the concentration of plasma. Binding of LPS-binding protein (LBP) in plasma to immobilized E. coli-LPS was inhibited by pre-incubation of plasma with LA-14-PP or Rs-LPS. Together with the result that treatment of plasma with anti-LBP antibody abolished the cofactor activity of plasma, these results indicated that LA-14-PP and Rs-LPS depleted LBP from plasma, resulting in inability of LPS to act on neutrophils. Thus LA-14-PP and Rs-LPS inhibited the action of LPS on neutrophils by at least two mechanisms, blocking of LPS receptor recognition and depletion of the cofactor LBP.