Mucociliary transport is an important nonimmunological defense mechanism of the respiratory tract. The aim of this study was to investigate the effect of sulfur dioxide (S02) at different concentrations on ciliary beat frequency (CBF). Ciliated cells were obtained from 12 volunteers by nose brush. CBF was quantified using video-interference microscopy. The cells were placed on a polycarbonate membrane in contact with the surface of a reservoir filled with RPMI 1640 (bicarbonate buffered) or Ringer's (electrolyte) solution, allowing the cells to be supplied by capillarity. In an exposure chamber the cells were exposed for 30 min to S02 2.5-12.5 ppm at 37°C and 100% air humidity. S02 induced a dose-dependent decrease in CBF of the cells cultured in Ringer's solution. S02 at 2.5 ppm caused a 42.8% decrease and at 12.5ppm a 96.5% decrease (8.1 + 0.24 versus 0.28 + 0.20 Hz). CBF of cells cultured in RPMI 1640 was reduced only moderately after 12.5 ppm S02 exposure (7.9 + 0.26 versus 6.70 + 0.30 Hz). In Ringer's solution a decrease in pH was observed after 30 min of S02 exposure to 12.5 ppm to a minimum value of 3.6. By contrast, the pH of RPMI 1640 remained constant at 7.5 under identical conditions. After adding RPMI 1640 to Ringer's solution, CBF increased in parallel to the pH to control values (5.0ppm: 4.64 + 0.45 to 8.51 + 0.60Hz). These data suggest that the highly water-soluble S02 reversibly eliminates CBF in correlation with a decrease in pH.