The clearance rate of nucleated pigeon erythrocytes injected intravenously into mice and rats has been calculated either by routine differential red cell counts or by measuring the radioactivity of the erythrocytes tagged with P32. Histological evidence is given that the foreign erythrocytes are phagocytized by the reticulo-endothelial cells of the liver and spleen. The clearance rate of the foreign erythrocytes, which measures the speed of the phagocytosis, follows in mice a regular exponential function similar to this previously established for other colloids. No spontaneous antibodies to pigeon erythrocytes could be detected in mice. The rapid and complex clearance rate of pigeon erythrocytes observed in rats is related to the existence of spontaneous specific antibodies. The simultaneous injection of pigeon erythrocytes and of India ink into mice, both phagocytized by the RE cells, results in a competition between the two substances in favor of the smaller particles of carbon.