Ventilatory function and whole lung mucociliary clearance have been assessed in 10 patients with mild stable asthma following inhalation of 1 mg of the beta-adrenergic receptor agonist terbutaline sulphate (Bricanyl, Astra Pharmaceuticals) from a metered dose inhaler (MDI). Compared to placebo inhalation, terbutaline produced marked bronchodilatation (mean percentage increase in FEV1 14%, P less than 0.01). Mucociliary clearance (measured by the in vivo radioaerosol technique) was assessed on three occasions--control, followed by placebo or terbutaline studies in a double-blind, cross-over manner. Particles were removed from the lung at a similar rate in all three studies. The mean (+/- s.e. mean) percentage of aerosol retained in the lungs after 6 h was 58 +/- 5%, 57% +/- 5% and 57 +/- 4% for control, placebo and drug studies respectively. It is concluded that terbutaline sulphate, given as a 1 mg acute dose, does not enhance mucociliary clearance in mild stable asthmatics, although it produces marked bronchodilatation.