Seven volunteers inhaled monodisperse 1.2 and 3.9 μm dia. (1.9 and 6.1 μm equivalent aerodynamic diameter) fused aluminosilicate particles labelled with 85Sr and 88Y respectively. Lung retention has been followed for over 200 days. Approximately 8% of 1.2 μm and 40% of 3.9 μm particles cleared within 6 days. It was calculated that these fractions were deposited within the conducting airways and therefore no rapid alveolar clearance was observed. Overall retention of the remaining material showed half-times averaging 320 days, with considerable intersubject variation but little effect of particle size. Two-component exponential functions represented retention more satisfactorily. Approximately 4% of 1.2 μm and 11% of 3.9 μm particles retained at 6 days cleared with a half-time of about 20 days, the rest with half-times of 330 and 420 days respectively. Estimated lung dissolution rates were 1×10-3 and 5×10-4 d-1 for 1.2 and 3.9 μm particles, indicating that for both sizes mechanical clearance shows a slow phase with a half-time of about 600 days.