Temporal course of postnatal changes in the gamma isoform composition of human fetal haemoglobin (HbF) was studied in 259 cord, 272 infantile and 216 adult Japanese blood samples. Reversed phase high-performance liquid chromatography was used for determination of the three isoforms of the gamma chain (A gamma T, A gamma I and G gamma), and the adult samples, usually with less than 1% of Hb F, were enriched for Hb F by an alkali denaturation-salting out procedure. The results show that the G gamma gene expression, kept at a higher and strictly-controlled level in the neonatal period, undergoes a gradual change during 1 year, beginning 3-4 months after birth, to the adult stage which is characterized by a generally lower and loosely-controlled expression of the gene. Further change seems to occur after 1 year, settling to the final adult level. The A gamma T gene frequency is estimated as 0.139 in the present Japanese adult population, and is essentially identical to the value for the newborns in the same population (0.141).