» Krypton Kr 81m is a gas suitable only for inhalation in diagnostic studies, and is obtained from a generator that contains rubidium 81 adsorbed on an immobilized suitable column support. Rubidium 81 decays with a half-life of 4.58 hours and forms its radioactive daughter ^81mKr, which is eluted from the generator by passage of humidified oxygen or air through the column. Rubidium 81 is produced in an accelerator by proton bombardment of Kr 82. Other radioisotopes of rubidium are produced and are present on the generator column. These other radioisotopes do not decay to ^81mKr. The column contains not less than 90.0 percent and not more than 110.0 percent of the labeled amount of Rb 81 at the date and time indicated in the labeling, and on elution yields not less than 80.0 percent of ^81mKr.