Changes in the free-cell population of the lungs of two strains of mice (SAS/4 and CBA/H) were studied up to 4 months after inhalation exposure to a sized fraction of 239PuO2 particles (1.5 micron AMAD) to give initial alveolar depositions (IADs) ranging from 17 to 810 Bq. A sample of the free-cell population of the lung was recovered by bronchoalveolar lavage, and a radiometric method was used to estimate the total number of pulmonary alveolar macrophages (PAM) in the lung. The response of the lung to 239PuO2 was characterized by an initial, dose-dependent depression in the total number of PAM following an IAD as low as 50 Bq. At IADs greater than 150 Bq, the initial depression continued for longer, merging into a chronic phase in which the PAM were larger and were accompanied by a minor infiltration of leukocytes. These findings were confirmed by histology, which also revealed focal accumulations of Type II pneumocytes. The results indicate that inhaled alpha-emitting particles are effective at producing a depletion in the alveolar macrophage population at relatively low IADs and that chronic effects on the cells can be produced by higher concentrations.