Human chest clearance of Teflon particles with an aerodynamic diameter of 4.7 micrometers tagged with 198Au or 111In was studied with two apparatuses and two gamma-ray spectrometers for the external detection of the activity deposited in the respiratory tract. Approximately the same chest retention function was measured with two gamma-ray spectrometers when the subjects inhaled equal aerosols under equal breathing conditions. The long-term clearance rate following the short-term elimination of particles from ciliated airways was slower for Teflon particles (mean half-time 105 days for 111In-labeled particles and 128 days for 198Au-labeled particles) than for iron oxide particles of the same size (mean half-time about 60 days). It is suggested that insoluble particles of this size studied are cleared with a half-time of about 120 days within the first 2 weeks after completion of mucociliary clearance. Regional deposition did not differ between the iron oxide and Teflon particles.