We briefly review bronchial mucociliary clearance: one of the non-specific host defence mechanisms operating in the lungs, helping to maintain cleanliness and sterility. We describe in detail the measurement in man of bronchial mucociliary clearance by means of the radioaerosol technique used in our laboratory. We report the use of the technique for detecting impairment of tracheobronchial clearance in patients with chronic bronchitis compared with healthy control subjects. We illustrate whole lung and tracheobronchial retention curves in a patient with Kartagener's syndrome - and also preliminary results on the inter-subject variability of clearance in a small group of patients (n = 5) with this syndrome. In 24 chronic bronchitics studied on three separate occasions intra-subject variability of 6 hr tracheobronchial clearance was 17%, inter-subject variability was 25% - figures commensurate with those reported by other authors.