Male golden hamsters were exposed to coal fly ash (FA) at the concentration of 0 (control), 1, 2, or 20 mg/m3 for 3 or 6 months (20 hr/day, 7 days/week). They were sacrificed at 0, 1, 3 and 6 months after exposure to undergo morphological investigation of the lung clearance of particles ingested by alveolar macrophages (AMs) following excessive deposition of FA particles. Particle-laden AMs in a cluster decreased in inverse proportion to the dust burden after the cessation of exposure. AMs with well-spread lamellipodia frequently appeared on the degenerated AMs in a cluster. In addition, neutrophils also agglomerated mainly in the alveoli proximal to the terminal bronchioles and ingested a small amount of particles. These results suggest that AMs and neutrophils might recruit into alveoli enclosing AMs in necrosis and reingest released particles. Macrophages in bronchioles generally ingested fewer particles than those clustered in alveoli. This finding suggests that AMs ingesting a small amount of released particles could move on the mucociliary escalator of the terminal bronchioles. On the other hand, AMs ingesting a large amount of particles would agglomerate in alveoli due to the loss of their migrating ability. The latter agglomeration of particle-laden AMs might be responsible for the retardation of lung clearance in proportion to the lung burden of particles.