OUR OVERALL HYPOTHESIS is that post-transfusion survival of allogeneic and autologous RBCs can be accurately quantified in anemic human infants using biotin-labeled RBCs combined with mathematical modeling that adjusts for confounding factors commonly encountered in neonates. These confounding factors include 1) dilution of labeled RBC as a result of growth stimulated erythropoiesis, anemia stimulated erythropoiesis, and blood transfusion; 2) loss of labeled RBC due to laboratory phlebotomy; and 3) variable RBC life spans resulting from RBCs having been produced at different developmental periods and under varying rates of erythropoiesis. In contrast to infants, adjustment for these factors is not necessary in healthy adults under conditions of steady state erythropoiesis. Instead in adults, RBC survival is typified by a linear decline in concentration of labeled RBCs over time. When this line is extrapolated to zero concentration, the intercept with the time axis represents the mean potential lifespan (MPL) of RBCs. (<7 d) and stored (>21 d) allogeneic adult RBCs transfused in the same infant.