The effect of sulfur dioxide (SO2) exposure on local bronchial sensitization to inhaled antigen was studied in the guinea pig. Exposure to SO2 (0.1 to 16.6 ppm) was performed in a 20 L exposure chamber for 8 hours on 5 consecutive days, while temperature, moisture, and concentration of SO2 were monitored and kept constant. SO2 concentrations were measured hourly by Schiff's reaction. On the last 3 days, SO2 exposure was followed by inhalation of nebulized ovalbumin (OA) for 45 minutes. One week later, specific bronchial provocation with inhaled OA (0.1%) followed by plethysmographic measurements of airway obstruction were performed every 2 days during a 2-week period. Specific antibodies against OA were measured in serum and bronchoalveolar fluid by a direct enzyme immunoassay. The SO2-exposed group (N = 17) demonstrated 67% to 100% positive bronchial reactions to inhaled OA, depending on the concentration of SO2, whereas the control group without previous SO2 exposure (N = 14) demonstrated bronchial reactions in only one animal (7%: p less than 0.05). The degree of bronchial obstruction was significantly higher in the exposed group, compared to the control group, for all SO2 concentrations (p less than 0.05). OA-specific antibodies in serum and bronchoalveolar fluid increased in SO2-exposed groups significantly, compared to the control group (p less than 0.05). It is concluded from these results that exposure to SO2 in low and medium concentrations can facilitate local allergic sensitization in the guinea pig.