The effect of vibration (41.0 +/- 5.4 Hz; 2 mm amplitude) on the clearance of lung secretions was ascertained in 10 patients with chronic bronchitis who complained of difficulty in raising sputum. Vibration was applied to the back for one hour by an electrically driven pad while the patients reclined on a couch with the trunk at 45 degrees to the vertical. Each patient had randomized control and experimental runs of 5 hours' duration that were identical in all respects except for vibration. The rate of clearance of secretions from the lung was assessed by serial whole lung counts after inhaling monodispersed tracer particles tagged with a gamma-emitting radioisotope (technetium-99m) and by sputum production. The mean rates of clearance and of sputum production were slightly higher during the vibration runs than the control runs but the differences were not significant.