Air pollutants were found to enhance the allergic sensitization of mice to an inhaled antigen. Aerosolized ovalbumin was used to mimic the inhalation of an environmental allergen. In three experiments the antigenic contact was repeated at 4 to 7 times over a period of approximately a month. Groups of mice were intermittently exposed to ozone at 0.5 and 0.8 ppm, sulfuric acid aerosol (1 mg/m3), and a combination of the two air pollutants. Antigenically sensitized mice showed some evidence of atopic reactivity to the inhaled antigen, but the interpretation of these responses was difficult to evaluate by observation alone. Clear evidence of allergic sensitization was obtained by injecting the antigen intravenously and recording the instances of systemic anaphylaxis. Allergic mice demonstrated anaphylactic shock within a few minutes of the injection, and fatally shocked animals died within 20 to 40 min. Significant increases in the levels of sensitization were obtained in animals exposed to ozone and the combination of ozone and sulfuric acid aerosol.