A standardized method is described to determine the mucociliary clearance of monodisperse insoluble 111In labelled polystyrene particles in the upper airways of anesthetized dogs. 60 μl of particle suspension are dispersed through a nozzle at the tip of a fiberoptic bronchoscope. The particle transport in the trachea is followed by the radioactivity movement using a gamma camera. The results for 13 dogs indicated an intraindividual subject reproducibility of about 25%.