Studies were carried out during the 1st yr of life in normal infants born at term to determine the proportions of fetal hemoglobin (Hb F) and adult hemoglobin (Hb A) being synthesized, in order to describe the complete switchover from Hb F to Hb A synthesis during postnatal life. 53 blood samples from 37 infants were incubated in an amino acid mixture containing [14C]leucine and chromatographed on DEAE-Sephadex for separation of Hb F and Hb A fractions. The completeness of the CEAE-Sephadex separation of Hb A and Hb F at an age when the major portion of synthesis was of the adult type of hemoglobin was confirmed by globin chain chromatography with the use of carboxylmethyl cellulose. There was a rapid decline in Hb F synthesis postnatally until 16-20 wk of age when levels of 3.2% plus or minus SD 2.1% were reached. By combining this data with that previously published, the complete switchover from Hb F to Hb A synthesis can be described in humans in relation to postconceptional age. It follows a sigmoid curve; the steep portion, which lies between the 30th and 52nd postconceptional week, is preceded and follwoed by plateaus averaging 95% and 7% Hb F synthesis, respectively.