It is proposed that intra-arterial injection of suitably sized large particles of radioisotopes may be a useful method of producing localized radiation effects, and data are provided to demonstrate this feasibility. Most of the investigations were made with Y/sup 90/, in the particle size range between 50 and 100 . Particles of appropriate size were prepared from pure Y/ sub 2/O/sub 3/, which was pelleted by a hydraulic press, and the pellets mounted in a furnace and partially fused at about 2800 un. Concent 85% C. The fused portion was then crushed and screened through sieves to eliminate particles larger and smaller than the desired size. Each sample was then irradiated at 8 x 10/sup 12/ neutrons per cm/sup 2/ or 4 x 10/sup 13/ for from 14 to 30 days. Injections into the portal vein in rabbits showed that 100% of the particles were retained by the liver; there was homogeneous distribution throughout the organ, and no histologic damage was produced. In dogs receiving varying quantities of Y/ sup 90/ suspensions intravenously or into carotid artery, there was little trapping of isotope by the brain and there was no apparent brain damage, demonstrated clinically, histologically, or by electroencephalograms. Dogs tolerated intravenously up to 6 mC/kg of body wt. and were still alive in good health 7 months after injection. From 40 to 70% of the particles were trapped in the kidney or thigh muscles when the suspensions were injected into a renal artery or into a femoral artery of a rabbit. Ten to 25% of the amount passing through rabbit kidney or thigh was trapped in the lungs: the rest went to liver and spleen. After intravenous injections in dogs st run. Concent 70% was trapped in lung, and the rest went to the liver and spleen. Results from treatment of 12 cancer patients with injections of Y/sup 90/ particles showed that; the procedures were relatively simple; there were no complications or toxic effects due to radioisotopes; no patient had radiation sickness; the procedures were also relatively safe to hospital personnel; there was little, if any, spill of radioactivity in excreta; and there was a favorable concentration of Y/sup 90/ in cancerous lungs and pelvic organs and there was immediate objective and subjective improvement in all cases. Localization in the liver was not so good as anticipated since there were considerable mechanical difficulties in injecting the material directly into the hepatic artery. (TCO)