The amount of plutonium transported from the lungs to blood following the intubation of mixed oxide suspensions obtained from a polydisperse aerosol depends primarily on the presence of plutonium dioxide (PuO2) particles of about 0.001m diameter. These particles under certain conditions may represent up to 50% of the total plutonium. The extra-pulmonary tissue distribution of plutonium is similar to that obtained for soluble plutonium citrate. The enhanced excretion of plutonium in the early phase following the administration of the 0.001 -μ m diameter particles is due in part to a low molecular weight species identified in blood and which is capable of filtration by the kidneys. The observations reported here indicate that the Task Group Lung Model is not applicable to aerosols containing 0.001-μm diameter particles if these are present in significant quantity. Furthermore, the use of an empirical fixed factor to determine systemic burden from urine analysis data is not valid.