The mechanism of development of plutonium-induced pulmonary sclerosis following the inhalation of soluble plutonium compounds was studied by light and electron microscopy and with biochemical methods. Plutonium-induced pulmonary sclerosis is a consequence of radiation injury. The effects seen are due to excessive reparative functioning of connectivetissue when disturbance of physiological regeneration takes place, and due to intensified destruction of parenchyma as a result of radiation injury. Endothelial elements of capillaries show the greatest susceptibility to radiation damage. Interalveolar sclerosis develops as a consequence of intensified proliferation of fibroblasts. Collagen fibers are formed in the basal layer of the alveolar-capillary barrier and in positions previously occupied by capillaries. Infection and the consequent aggravation of pneumonic processes is a significant factor in development of sclerosis.