The catabolism and distribution of simultaneously injected rabbit transferrin and rabbit asialotransferrin was studied in five rabbits using a dual-isotope tracer technique. Similar studies were performed with human transferrin and human asialotransferrin in two humans, seven rabbits, and a rat.In all experiments, the catabolic rate of the asialotransferrin was higher than the corresponding value for the control protein. The difference averaged at 15% for rabbits and 29% for humans when homologous protein preparations were used. There were no concomitant differences in capillary transfer rates or in the partition of the labeled proteins between intra- and extravascular spaces.The catabolic rate of human transferrin in rabbits was not significantly different from that of the corresponding homologous protein. In contrast, human transferrin from which 92–100% of the sialyl residues had been removed was catabolized in rabbits at approximately 3.5 times the rate for homologous asialotransferrin. Substantial difference was found between the elimination of human control and asialotransferrins in the rat as well.