Studies of fetal hemoglobin (HbF) in the blood of infants and children, together with some observations of adult hemoglobin (HbA) in adults, suggest the following: HbA and HbF may be present in the same erythrocyte. The HbF concentration in erythrocytes is high at birth but decreases to low levels by 6 months of age. The postnatal decrease in HbF is due largely to the production of cells containing mostly or perhaps entirely HbA. HbF and HbA are not formed in separate cell clones. All erythropoietic cells have the capacity to produce both fetal and adult hemoglobin.