Summary (1) Following in vitro labeling of erythrocytes with Na2Cr51O4, a considerable amount of the Cr51 was found unattached to hemoglobin. (2) This unattached Cr51 lost the ability to label erythrocytes, and when given intravenously, was excreted rapidly by the kidneys. (3) Hemoglobin-bound Cr51 was cleared from the blood the same as was previously reported with I131-hemoglobin. After clearance, however, much tissue retention of Cr51 was noted. (4) Rats metabolized homologous and canine hemoglobin similarly, with slow excretion of Cr51 by the kidneys. (5) The clinical implications of these findings indicating some peculiar metabolic characteristic of Cr51-tagged erythrocytes are discussed.