AT least two forms of adult haemoglobin are present in the blood of the normal human adult, a main component haemoglobin A1 and a minor one haemoglobin A2 (ref. 1). In normal individuals the concentration of haemoglobin A2 is about 2.5 per cent of the total haemoglobin. In persons heterozygous for the Cooley gene, this amount is increased to a level of about 5 per cent. The A2 fraction can be isolated from the main component by starch-block electrophoresis and by chromatography on carboxymethyl cellulose2. The presence of two haemoglobin A2 fractions in some persons, belonging to different generations of one family, makes it likely that the synthesis of haemoglobin A2 is genetically determined2. This led us to investigate whether there exist chemical differences between the main component, haemoglobin A1, and haemoglobin A2, and to locate eventual differences.