Promotion of lung tumor formation from inhaled 239 PuO 2 in rats may be associated with aggregation of plutonium particles near bronchioles. The relationship of plutonium particle aggregation in the lung and the development of lung tumors after inhalation of 239 PuO 2 was studied in 664 life span rats with mean lung doses ranging from 0.35 to 20 Gy. Plutonium particle concentration and aggregation were determined from autoradiographic sections of the left lung lobe. The increase in particles/cm 2 and mean number of particles per aggregate up to 20 Gy were directly proportional to lung dose. Aggregates with >25 particles increased linearly with dose from 0.2% at 1.4 Gy to 8.2% at 20 Gy, in a pattern similar to increasing severity of pulmonary fibrosis and incidence of lung tumors. Lung tumor incidence increased from about 6% at 1.4 Gy to 83% at 8 Gy; no further increase in lung tumors was seen at doses >8 Gy. Maximum lung tumor incidence at 8 Gy corresponded to a particle concentration of 130/cm 2 and four particles/aggregate with 4% of aggregates having >25 particles. Aggregation of inhaled plutonium particles in clusters of >25 particles resulted in daily doses of only a few centigray to focal tissue regions containing clustered particles, yet these doses appeared sufficient to cause pulmonary fibrosis and promotion of pulmonary carcinogenesis.