IN a study1 of the haemoglobins of Macaca irus obtained from Thailand and Vietnam two α-chain variants, electrophoretically distinguishable from the normal type (Hb-Ami), were found to be frequent. One, Hb-Qmi, was anodal to Hb-Ami on starch gel at pH 8.6 and varied considerably in relative concentration when present with the latter. The fingerprint pattern of the soluble tryptic peptides was, however, indistinguishable from that of Hb-Ami and the difference is presumably in the insoluble peptide core. The other variant, Hb-Pmi, had the same electrophoretic mobility as Hb-Qmi in very fresh haemolysates but formed aggregates with ageing so that it then appeared in starch gels as a trail extending cathodally from the Hb-Ami position. The fingerprint of Hb-Pmi was distinct from that of Hb-A mi, αT3 (tryptophan positive) and αT4 (histidine positive) being absent and a new tryptophan and histidine positive peptide appearing in the neutral region. A single amino-acid substitution in α15 or 16 would probably account for this. The phenotype frequencies of these two variants in the population were consistent with their being determined by alleles at the α-locus. In addition, about 20 per cent of the animals had a minor haemoglobin component. The distinctive chain of this component was shown2,3 to differ from the normal α-chain in at least four substitutions. In some animals the minor component was present together with Hb-Ami and either Hb-Pmi or Hb-Qmi and it was suggested that the characteristic chain arose by duplication of the α-locus and mutational divergence.