Summary: An attempt has been made to saturate the anti-kidney antibody-binding sites in the mouse kidney. The administration of even 15 mg of a globulin fraction of anti-mouse-kidney rabbit serum (G-anti-MK) did not saturate these sites, since this quantity of unlabeled anti-kidney antibody had no appreciable effect on the renal accumulation of radioactivity in animals that simultaneously were given radioiodinated G-anti-MK. A minimum value for the amount of antibody which can localize in the kidney was calculated, and was found to correspond to about 3 per cent of the weight of a mouse's glomeruli. When large amounts (about 15 mg) of either G-anti-MK or globulin fraction of anti-mouse-spleen rabbit serum (G-anti-MS) were given along with radioiodinated G-anti-MK there occurred an unusually rapid decline in the blood levels of radioactivity, indicating a physiological effect of these sera.