Although exposure of animals to ozone (O3) has been shown to impair host defense mechanisms and to induce inflammation, very little is known about its inflammatory effects in humans. In the study, 11 healthy males, age 18-35, were exposed to 0.4 ppm O3 or filtered air for two hours with intermittent exercise. Eighteen hours later, bronchoalveolar lavage (BAL) was performed. There was an 8.2-fold increase in the number of polymorphonuclear leukocytes (PMNs), and a corresponding decrease in the number of macrophages, indicative of an inflammatory response. No significant change in the number of lymphocytes was observed. A two-fold increase in the levels of classical indicators of vascular permeability (protein and albumin) clearly indicated that an acute O3 exposure can increase permeability. A selected number of biochemical markers known to be associated with inflammation were examined. The results of the study show that an acute exposure to O3 can result in increased levels of inflammatory materials capable of producing pathophysiological changes in the lower airways of humans.