The number and functions of macrophages in the lungs are crucial factors for prevention and development of lung disease caused by inhaled particles. To examine whether airway macrophages are attracted to the site of particle deposition and what proportion of these macrophages is involved in phagocytosis, aerosols of 6-microns polystyrene particles (PSP) were inhaled by Syrian Golden hamsters under controlled conditions through an inhalation tubule and their lungs were fixed by intravascular perfusion within 20 min (PSP-1, PSP-1a), 40 min (PSP-2), and 24 h (PSP-3) after the beginning of the inhalation. The number and the phagocytic activity of airway macrophages were studied in situ with a fractionator, a stereologic method, on light microscopic sections. No significant increase in macrophage number was detected for the groups PSP-1 and PSP-1a. The increase for group PSP-2 was, however, between 2- and 3-fold, whereas for group PSP-3 the increase was between 1.5- and 2.5-fold with respect to control animals, which had inhaled ambient air through an intubation tubule (C-2) and whose lungs had been fixed after 40 min. There were no significant differences among the four groups with respect to the proportion of airway macrophages that had phagocytized polystyrene microspheres. Twelve to fifteen percent of the macrophages were found to be involved in phagocytosis. In the case of the mean number of particles per phagocytizing macrophage, there was a significant decrease for the PSP-3 group with respect to the pool of the three groups PSP-1, PSP-1a, and PSP-2 taken together. These studies demonstrate (1) that airway macrophages are rapidly recruited to the sites of particle deposition and (2) that only a small proportion of very active macrophages contributes to the clearance of particles, suggesting a great potential of airway macrophages to interact with many more particles than the hamsters were exposed to in this study.