Until recently, fetal hemoglobin was a protein which could be detected in human blood only during the first year of life. Therefore the interest in this hemoglobin was mainly limited to those who were interested in infant physiology. Since Vecchio's discovery that fetal hemoglobin is found in large amounts in the blood of patients with Cooley's anemia, the interest of hemotologists has increased. It is now known that also in patients with sickle-cell-syndrome anemia (and perhaps in other hereditary hemolytic anemias) fetal hemoglobin may occur. An accurate method for the determination of small percentages of fetal hemoglobin is