A new, nearly monodisperse human serum albumin particle produced by air-pressure-nebulization was inhaled by children and lung-transplant patients.After inhalation of the particles obtained with an air-pressure nebulizer, the initial deposition pattern showed a marked tracheobronchial deposition which could be reproducibly obtained without a special breathing technique, the alveolar deposition being not higher than 10%. With the use of 99mTc, the radiation exposure is limited to a level which is low enough for children, but images can be taken up to 24 hrs later. Further parameters for in vivo characterization of the mucociliary function are the 24 h retention pattern and the velocity of particle motion in the trachea.Within the first 45 minutes, the global clearance rate was 51% in healthy children, which is rather high in comparison with the literature, most likely due to size of particles and the selection of patients with a mean age of 10.7 years. In ciliary dysfunction, the initial clearance rate was 16% and 46% within 24 h.With simplification of the preparation, application, and examination technique, this method is to be used in children, so that a wider use can be anticipated. The normally fast initial elimination of particles allows quick differentiation of normal and impaired ciliary function.