The red blood cell population of the human newborn is more heterogeneous than that of adults. To characterize this heterogeneity, red cells were separated on Stractan gradients into populations according to their density and, therefore, generally according to their age. Lipid content, phospholipid asymmetry, osmotic fragility, and deformability were measured. With increasing density, osmotic resistance and deformability decreased to a greater extent in the neonatal than the adult red cell. There was a marked loss of surface area throughout the life span of the neonatal red cell which was not accompanied by a parallel loss of membrane lipid. This apparent discrepancy could be explained at least in part by internalization of membrane lipid as a result of endocytosis, a process shown to be present in even the densest neonatal red cells.