The best known label that can bind to erythrocytes and be used to monitor their survival in the circulation and thereby establish their life span is radioactive sodium chromate (51Cr). Chromium penetrates the red cell membrane and once inside the cell it binds to the hemoglobin. The other isotopes used in establishing a subject’s erythrocyte life span conduct their labeling by being incorporated into the hemoglobin molecule as it is synthesized in new cells. Chromium binds to erythrocytes in vivo as well as in vitro. The anionic (hexavalent) chromium in the form of the chromate ion (51CrO4 −2) passes through the red cell membrane and once it is within the cell it is converted to the trivalent cation (Cr +3) which binds to the globin moiety of hemoglobin. This form of the ion will not be reutilized and become incorporated into other red cells upon release from a labeled cell when the latter dies because it is unable to penetrate the plasmalemma of the red cell. It is excreted (in the urine) thus making some of the calculations more straightforward. When samples of erythrocytes are labeled in vitro ascorbic acid is added to the suspension at the end of the incubation period to convert any surplus chromate that has not been incorporated into the red cells to the reduced trivalent state. This prevents any further labeling of other erythrocytes when the sample is introduced into the animal. A potential disadvantage of this label is that its affinity for hemoglobin though strong is not irreversible. This so-called elution has been best recognized in human erythrocytes (in man approximately half of the radionuclide initially present leaves the circulating red cell after 55–90 days). The usual assumed rate of loss is about 1% per day but it can be higher in certain diseases in man. The degree of elution of 51Cr from labeled avian erythrocytes has not been assessed in particular detail. But in comparison with other labels employed to monitor the survival of chicken and duck erythrocytes, chromium is apparently not lost at a particularly great rate from these cells and its use seems to lead to the 239determination of life spans equivalent to those generated by alternative methods of study.