In addition to the biotin labeling of RBC method, the "differential agglutination, antigenic method" using flow cytometry will be applied for infants receiving allogeneic RBC transfusions. The results of the two methods will be compared with one another. The differential agglutination method utilizes differences in the RBC surface antigens between the donor and the recipient for determining short- and long-term RBC survival. Genotyping results of adult donor and the infant recipient RBC antigens (performed at the Mississippi Valley Regional Blood Center) permits identification of minor blood group differences between donor and recipient RBCs so that appropriately labeled minor RBC antibodies (available commercially and used by blood banks for minor blood group RBC typing) can be used in flow cytometric determination of RBC survival. RBC survival determined for each of the biotin labels and for the RBC antigenic differences will all be compared among one other. The differential agglutination/antigen method will considered the "gold standard" as the RBCs are labeled ex-vivo, AFTER the RBC transfusion, thus without modifying the RBC membrane surface proteins as biotinylation does. Our hypothesis is that the addition of too much biotin can lead to artifactually shortened in vivo RBC survival, but that lower doses of biotin to not. By also including the differential agglutination/antigen method RBC survival results, we will be able to better validate our multi-density biotin hypothesis.