SPECIFIC AIM #1 (Thrasher Foundation Grant): To develop in vitro and validate in vivo in adult humans and anemic infants the capability of biotinylating RBCs at up to 5 discrete densities for simultaneously determining RBC kinetics of multiple, distinct RBC populations. This requires expansion of the RBC biotin labeling technology we have previously developed. After refinement, the method will be applied in the subsequent aims conducted in anemic infants receiving clinically ordered RBC transfusions.