The short term effects on the free macrophage population in the mouse lung, following inhalation of thulium-170 oxide, have been studied, with a view to using the system as a model with which to compare the short term effects of inhomogeneous alpha irradiation. The clearance and distribution of thulium-170 was followed up to 44 days post inhalation. The mice had an initial lung burden (+/- SE) of 6.7 +/- 0.5 microCi 170Tm/g of lung and the pattern of removal of this could be described by a single exponential of half time 31 +/- 4 days. This exposure resulted in a uniform dose of about 24 Gy to the lung. The macrophage population was studied by counting cells lavaged from the lungs at intervals over the period of the experiment. The lung burden of 170Tm2O3 was found to decrease the free macrophage numbers by about a factor of 2 in 14 days with some slight recovery at 6 weeks post inhalation. It was found that nearly 50% of the activity in the lung lobes was removed by the lavages and that this fraction remained constant throughout.