We tested the hypothesis that exposure of healthy volunteers to concentrated ambient particles (CAPS) is associated with an influx of inflammatory cells into the lower respiratory tract. Thirty-eight volunteers were exposed to either filtered air or particles concentrated from the immediate environment of the Environmental Protection Agency (EPA) Human Studies Facility in Chapel Hill, North Carolina. Particle concentrations in the chamber during the exposures ranged from 23.1 to 311.1 μ g/m³. While in the exposure chamber, volunteers alternated between moderate exercise (15 min) and rest (15 min) for a total exposure time of 2 h. There were no symptoms noted by volunteers after the exposure. Similarly, there were no decrements in pulmonary function. Eighteen hours after exposure, analysis of cells and fluid obtained by bronchoalveolar lavage showed a mild increase in neutrophils in both the bronchial and alveolar fractions in those individuals exposed to CAPS (8.44 ± 1.99 and 4.20 ± 1.69%, respectively, in those with the greatest exposure) relative to filtered air (2.69 ± 0.55 and 0.75 ± 0.28%, respectively). Blood obtained 18 h after exposure to CAPS contained significantly more fibrinogen relative to samples obtained before exposure. We conclude that ambient air particles are capable of inducing a mild inflammation in the lower respiratory tract, as well as an increased concentration of blood fibrinogen.