In order to identify the fetal features in neonatal erythrocytes, cord blood was separated into seven fractions of varying specific density. Cell age in the single fractions was ascertained by means of reticulocyte count, glutamic-oxalacetic transaminase activity, and hemoglobin F concentration. The same procedures were used with blood of adults. With the exception of the fraction of neonatal blood with the highest specific density, the blood from neonates and adults correlated well for cell age and specific density. The highest specific density fraction of neonatal blood was found to contain a higher proportion of younger cells.The comparison of enzyme activities in the single fractions between neonates and adults showed that a high activity of glucose-6-phosphatedehydrogenase and enolase and a low activity of phosphofructokinase are typical fetal signs of neonatal cells.SpeculationThe changing constitution of red blood cells (RBC), which were observed during the first months of life may be a part of the differentiation process. Due to their good accessibility and their relatively simple metabolism, erythrocytes from neonates can serve as a goodin vivomodel that helps to clarify the process of differentiation.