The effect of Legionella pneumophila sonic extract on human neutrophil and monocyte oxidative burst was studied by superoxide anion release and luminol-enhanced chemiluminescence assays. Legionella pneumophila sonic extract by itself did not stimulate neutrophils and monocytes. The sonic extract at 8-2000 micrograms/ml primed neutrophils for enhanced superoxide release and, at 8-62.5 micrograms/ml, for enhanced chemiluminescence. Monocytes were only primed for enhanced chemiluminescence at very low extract concentrations (below 16 micrograms/ml). Monocyte superoxide release was suppressed by extract concentrations higher than 2000 micrograms/ml and the chemiluminescence response of neutrophils and monocytes by concentrations higher than 250 and 125 micrograms/ml, respectively. The priming activity was heat stable and present in fractions below 5 kDa. On the basis of these findings it is suggested that enhanced production of oxygen metabolites by neutrophils in contact with legionella components at low concentrations could contribute to the lung tissue damage seen in Legionnaires' disease, whereas the suppression of phagocyte oxidative burst by higher extract concentrations may be one of the mechanisms by which Legionella pneumophila survives intracellularly.