SummaryA haemolysate of red blood cells of the grey kangaroo, M. giganteu8, gave several haemoglobin peaks when chromatographed on polycarboxylic-acid-type ion-exchange resins. In the four samples studied two major components, haemoglobin I and II, were present. In the sample examined in detail these comprised 51 and 45% respectively, together with about 4% of minor haem-containing proteins. The two major haemoglobin components were readily separated by chromatography at pH 6·4 in phosphate buffer of 0 ·152M sodium-potassium ion concentration.After removal of the haem the whole globin gave three components on chromatography on carboxymethyl cellulose in 8M urea-thiol buffers, whereas each purified haemoglobin treated in the same way gave only two components. The difference between the two haemoglobins of kangaroo was shown by amino acid analysis to be in the ,B-globin chains which differed by a single amino acid residue, a histidine residue in the ,B-II chain being replaced by a glutamine residue in ,B-1. N-Terminal sequences of 10 residues were established by the Edman degradative procedure, and the locus of this difference shown to be in residue 2. Amino acid analyses revealed that the length of the globin chains was the same as in human globin.The f3-chains contain a single methionine residue which is not present in the ",-chains. Oleavage at the methionine residue of the f3-chains with cyanogen bromide and separation of the two fragments has shown that the methionine is at residue 55, the same as in human and most animal ,B-chains.