Human neutrophils produce small amounts of superoxide anion when stimulated with the chemotactic peptide FMLP; preincubating neutrophils with low concentrations of lipopolysaccharides (LPS) markedly increases this response, an effect referred to as priming. In this work LPS from Coxiella burnetii either phase I (virulent) or phase II (avirulent) were examined for their ability to induce priming. Results clearly show that only LPS from phase II microorganism was able to increase the release from neutrophils upon subsequent stimulation with FMLP. This effect was abolished by preincubation of LPS with polymyxin B. This finding may account for the ability of Coxiella burnetii phase I to escape intracellular phagocyte killing during persistent infections.