THE haemoglobin of the fœtus and the new-born child and the haemoglobin of older children and adult people can be distinguished by their different resistance against 0·1 N sodium hydroxide. At pH 12 normal haemoglobin is denatured very rapidly, whereas foetal haemoglobin is denatured only slowly. By using this difference in denaturating-rate, it is possible to calculate the amounts of foetal and later haemoglobin present in small samples of blood1. A normal new-born child has only fœtal haemoglobin. During the early months after birth, this haemoglobin is gradually replaced by later haemoglobin, at a rate of a little less than 1 per cent a day, so that after 120 days normally all foetal haemoglobin has disappeared. In prematurely born babies, later haemoglobin appears only at the time that the child ought to have been born2.