Regional pulmonary arterial blood flow may be effectively measured by the distribution of radioactive particles in the lung following an intravenous injection. The original method required use of iodinated I 131 serum albumin, aggregated (MAA 131 [Human]), which limited the total amount of injected radioactivity to a maximum of about 300μc. To increase the photon output while decreasing the radiation-absorbed dose, we labeled iron hydroxide aggregates with the short-lived nuclide, indium In 113m. This isotope may be obtained economically in the carrier-free state from a tin Sn 113-indium In 113m generator in the clinical radioisotope laboratory. The preparation of the particles takes 15 minutes, and they may be autoclaved prior to injection. These aggregates have suitable physical and biological characteristics for lung scanning, and permit better images in a shorter period of time than iodinated I 131 serum albumin, aggregated.