Summary: When the radioiodinated globulin fraction of anti-rat-kidney or anti-rat-lung serum is injected into rats, there is some localization of radioantibodies in other than the homologous tissues or organs. The radioantibodies localizing in the different organs show differences depending upon the organ in which they localize. The iodinated antibodies from anti-rat-kidney serum which localized in the kidneys, liver and lungs of donor rats were eluted with sodium hydroxide from the insoluble sediments obtained from the homogenates of the tissues of the donor rats, and these purified antibody preparations were tested for localization properties by injection into recipient rats. It was found that the purified radioantibodies from all three tissues localized to some extent in the kidney, but the antibody isolated from each organ also showed some preference for localization in the organ of initial localization. Liver-isolated anti-kidney antibody localized in the kidney and in the liver, but to a greater extent in the liver than did kidney-isolated antibody. Likewise, lung-isolated anti-kidney antibody localized to a greater extent in the lung than kidney-isolated antibody. Radioiodinated antibody from anti-rat-lung serum was similarly purified from the kidneys, liver and lungs of donor rats, and here again the three preparations localized to some extent in the lungs of recipient rats, but the antibody isolated from each organ again showed a preference also for the organ of initial localization. From the very rapid rates of localization of the antibodies it has appeared that the localization must take place on the vascular bed of the organ, and that the fractionation by heterologous organs of the antibodies prepared against one organ demonstrates serological specificities or differences in the vascular beds of these organs. It is suggested that if certain perivascular diseases, such as glomerulonephritis or multiple sclerosis, are caused by a process of autoimmunization, it is probably the localizing antibody which is the damaging agent and the organ which is attacked is determined by the specificity of the vascular bed of that tissue.