Purification, in vivo, of the kidney-localizing antibody from radioiodinated anti-mouse-kidney serum has been accomplished by injecting the antiserum into mice, eluting the localized radioantibody from the perfused kidneys of these animals with alkali, and injecting the solution of recovered radioantibody into recipient animals. About 8 per cent of the radioactivity thus injected localized in the kidneys, whereas only ½ to 1% localizes from radioantiserum not specifically purified. Localization was rapid; and the ratio of radioactivity in tissue to that in blood was about 4 within three hours. The possible application of this procedure to the investigation of active glomerulonephritis is discussed.