Acute exposure of humans to 0.4 ppm ozone is known to cause production of components which mediate inflammation and damage in the lung. The contribution of alveolar macrophages to this process is not well understood. In addition, ozone may cause more extensive cellular changes than those currently measured by enzymatic or immunological methods. Therefore the authors have used molecular techniques to measure changes in the total spectrum of alveolar macrophage proteins in humans exposed to ozone. In the study, eight human volunteers were exposed once to 0.4 ppm and once to filtered air for 2 hours with intermittent exercise. Eighteen hours later bronchoalveolar lavage was performed and alveolar macrophages were isolated. Changes in proteins made by these cells after air or ozone exposure were analyzed by high resolution two-dimensional gel electrophoresis, using computerized densitometry to quantify changes in individual proteins. Of the nearly 900 proteins analyzed, 23 (2.6%) were synthesized at a significantly increased rate following ozone exposure while 71 (8.1%) were synthesized at a significantly reduced rate. These results indicate that exposure of humans to ozone causes extensive changes in the spectrum of macrophage proteins being produced. Quantitative two-dimensional gel electrophoresis is a highly sensitive technique which may reveal much more information about the in vivo effects of a pollutant than has previously been available. Furthermore the ability to survey large numbers of macrophage proteins after exposure to various inhaled pollutants may allow a better understanding of the mechanisms of action of these agents, as well as provide new biomarkers of pollutant exposure.