We recently reported that baseline expression of circulating CD11b is associated with the magnitude of the neutrophil response following inhaled endotoxin. In this study, we examined whether circulating CD11b plays a similar role in the inflammatory response following inhaled ozone exposure. Twenty-two volunteers underwent controlled exposure to ozone (0.4 ppm, 2 h) and to clean air on two separate occasions. Induced sputum and peripheral blood were collected before and after exposure. Induced sputum collected from subjects exposed to ozone revealed marked neutrophilia and increased expression of mCD14 on airway macrophages and monocytes. Baseline CD11b expression on blood phagocytes correlated positively with ozone-induced neutrophil influx into the airways. In conclusion, in human volunteers, circulating CD11b predicts the magnitude of the airway neutrophil response following inhaled ozone exposure. Consequently, CD11b may be a useful biomarker for predicting susceptibility to airway neutrophilic inflammation caused by pollutants.