The rate of phagocytosis of 59Fe-labeled erythrocytes of various ages was measured in the rabbit. Liver, spleen and the remaining parts of the reticulohistiocytic system (RHS) were compared with each other. The phagocytic activity in the liver increased from 66% in 4-day-old to nearly 90% in 70-day-old erythrocytes, whereas the radioactivity decreased in the other parts of the RHS from 33 to 3%. In the spleen, the radioactivity increased from 1.7 to 7.6%. On the 8th day, the radioactivity in the liver temporarily amounted to 92%, reducing that of the other organs. Comparison of the phagocytic activity per gram showed a ratio of liver to spleen of 1:2 after 4 days which changed to 1:5 after 70 days.