Beagle dogs received a single inhalation exposure to a monodisperse or a polydisperse aerosol of 241AmO2 to determine the radiation dose to internal organs. Three monodisperse aerosol sizes (0.75, 1.5 and 3.0 micrometer activity median aerodynamic diameter) and one polydisperse aerosol (1.8 micrometer activity median aerodynamic diameter) were used to assess the role of particle size in clearance of 241AmO2 from the lung and the rate of translocation of 241Am to other tissues. Dogs were sacrificed in groups of two or three at 8, 32, 64, 128, 256, 365 and 730 days after exposure. The clearance rate of 241Am from lung was particle size dependent; this resulted in an increased radiation dose to lung at 2 yr by 70% as particle size increased from 0.75 to 3.0 micrometer aerodynamic diameter. Conversely, the radiation dose to liver and skeleton decreased by 20 and 35% respectively as particle size increased from 0.75 to 3.0 micrometer activity median aerodynamic diameter. When the radiation doses absorbed by lung, liver and skeleton were compared to those predicted by the ICRP 30 model, differences were noted.