IN the course of a survey of positron-emitting radioactive isotopes for potential use in external localization of human brain tumours1, zirconium-89 (half-life, 79 hr.) was studied. Positrons with a maximum energy of about 0.9 MeV. and gamma rays of 0.9 MeV. energy are emitted. Previous animal studies employing zirconium salt solutions in metal displacement therapy of radioactive plutonium poisoning indicated a rapid excretion of the zirconium in the urine2. However, other investigations in animals suggested that various zirconium salts, when injected intravenously, formed colloidal aggregates which differed in distribution according to their particle-size. The larger aggregates were not rapidly cleared from the blood3.