A novel method for exposing mammalian cells in vitro to air pollutants under conditions resembling those in vivo is described. Cells grown on membrane filters are nourished and protected from drying during exposure by perfusion of nutrient medium from the opposite surface of the filter. The cells are covered by a thin layer of fluid comparable in thickness to the mucus layer overlying the respiratory epithelium. Exposure to NO2 at levels commonly measured in the ambient air of urban settings (0.1-0.2 ppm) produces a pronounced cytotoxic effect after less than 4 h of pollutant encounter. It is proposed that this cell exposure system will have utility in the study of effects of airborne pollutants on mammalian cells. © 1978, American Chemical Society. All rights reserved.