Experiments with a bolus technique suggest that retained fractions in the airways are dependent on the geometric diameter of the particles. This view has been adopted by the International Commission on Radiological Protection (ICRP) in its new human respiratory tract model (HRTM). The aim of the present study was to test this view by the use of an inhalation technique, in which particles with an aerodynamic diameter of about 6 microns are inhaled extremely (0.05 l/s) and as a result, the particles are deposited mainly in small ciliated airways. Nine healthy subjects inhaled on one occasion monodisperse 111In-labelled polystyrene particles (geometric diameter 6.05 microns, aerodynamic diameter 6.2 microns) and on another occasion monodisperse 111In-labelled Teflon particles (geometric diameter 4.47 microns, aerodynamic diameter 6.5 microns). Both particles were inhaled at 0.045 L/s and radioactivity in the body was measured after 0, 24, 48, and 72 hours as well as after 1, 2, and, for some subjects, also 3 weeks. The retention in the lungs at 24 hours was slightly lower for the Teflon particles (47%) than for the polystyrene particles (51%). From earlier experimental data with different particle sizes as well as from predictions with theoretical lung models, this difference is reasonably explained by the somewhat larger aerodynamic diameter of the Teflon particles. Clearance of the 2 particle types between 1 day and 2 weeks was similar within each individual as well as in the whole group. The differences between the clearance of 4.5 microns and 6 microns geometric diameter particles observed in the present experiment are significantly different (P < .01) from the differences seen in earlier shallow bolus experiments as well as from the differences for such particles calculated with the HRTM, i.e., our experiment does not support the hypothesis that the fraction retained after 1 day is dependent on the geometric diameter in the size range studied.