The studies proposed here build on biotinylation and mathematical modeling methods that our research group has developed for accurately measuring RBC survival without exposing the infant to radioactivity. The biotin RBC labeling method is well suited to newborn infants because it permits the simultaneous tracking of multiple RBC populations on <10 µL blood. Survival of RBCs labeled at multiple biotin densities will be quantified using the standard RBC survival parameters, ie, post-transfusion short-term 24 h RBC recovery and long-term modeled RBC survival, ie, until 50 and 100% of biotin-labeled RBCs have disappeared from the circulation. The latter two measurements will be calculated using the required mathematical adjustments for growth, laboratory phlebotomy loss, and intervening RBC transfusions.