Deposition and clearance of inhaled particles of iron oxide labeled with (198)Au were studied in 19 normal subjects (10 nonsmokers and 9 smokers). For this purpose, monodisperse aerosols of particles with a 2 mu diameter were produced in a spinning disc atomizer. Thoracic counts and images with a scintillation camera were begun immediately after inhalation of the aerosol and continued for 6 hr. In all subjects, smokers and nonsmokers, the deposition of the particles was uniform throughout both lung fields, with approximately half of the particles deposited in the ciliated airways (tracheobronchial deposition) and half in the nonciliated airways (alveolar deposition). Tracheobronchial clearance in nonsmokers occurred immediately after inhalation, first at a fast rate for particles deposited in the largest and most central airways, and then at a slower rate for particles from the smaller and more peripheral airways. Photoscintigrams showed that the particles cleared steadily with no retention in any area. The general pattern of clearance may be likened to a model of multiple conveyor belts with speed increasing from the peripheral to the central airways in such a way as to prevent "particle jams" at airway confluence points. In smokers, tracheobronchial clearance was delayed for periods of 1-4 hr after inhalation. Furthermore, in contrast with the findings in nonsmokers, significant clearance was still occurring in many of the smokers in the 5th and 6th hr after inhalation. Also, photoscintigrams showed an abnormal accumulation of particles in the large airways several hours after inhalation of the aerosol.