In order to clarify the role of alveolar macrophages (alv Mφ) in the iron metabolism of the lung, ultrastructural findings of cells obtained by bronchoalveolar lavage were studied by transmission electron microscopy in rats after inhalation of saccharated iron oxide and welding fumes as well as a patient with welder's lung. Alv Mφ of rats which had inhaled saccharated iron oxide formed phagosomes and secondary lysosomes 4 hours after inhalation. The first day after inhalation, many ferritin particles were seen in cytosol and formed a diffuse patterns; 19% of alv Mφ had ferritin particles in a paracrystalin arrangement in the secondary lysosomes. Electron micrographs of alv Mφ at 3, 6, 14, and 28 days after inhalation showed an increase of the cells with ferritin particles. Alv Mφ of rats which had inhaled welding fumes phagocytized those iron particles and also synthesized ferritin particles in the cytosol and secondary lysosomes 6 days after inhalation. Numerous ferritin particles were seen in the cytosol and secondary lysosomes of alv Mφ from a patient with welder's lung as well. These results suggest that inhaled iron was phagocytized by alv Mφ and stored in the cytosol by binding with apoferritin which was synthesized by the ribosomes.