Background: During the last week of June 2008, central and northern California experienced thousands of forest and brush fires, giving rise to a week of severe fire-related particulate air pollution throughout the region. California experienced PM10.2.5 (particulate matter with mass median aerodynamic diameter > 2.5 ♂m to < 10 Mm; coarse) and PM2.5 (particulate matter with mass median aerodynamic diameter < 2.5 ♂m; fine) concentrations greatly in excess of the air quality standards and among the highest values reported at these stations since data have been collected. Objectives: These observations prompt a number of questions about the health impact of exposure to elevated levels of PM10.2.5 and PM2.5 and about the specific toxicity of PM arising from wildfires in this region Methods: Toxicity of PM10.2.5 and PM2.5 obtained during the time of peak concentrations of smoke in the air was determined with a mouse bioassay and compared with PM samples collected under normal conditions from the region during the month of June 2007. Results: Concentrations of PM were not only higher during the wildfire episodes, but the PM was much more toxic to the lung on an equal weight basis than was PM collected from normal ambient air in the region. Toxicity was manifested as increased neutrophils and protein in lung lavage and by histologic indicators of increased cell influx and edema in the lung. Conclusions: We conclude that the wildfire PM contains chemical components toxic to the lung, especially to alveolar macrophages, and they are more toxic to the lung than equal doses of PM collected from ambient air from the same region during a comparable season.