The definitive erythrocytes of adult White Leghorn chickens contain two haemoglobins, a major component, A, and a minor component, D. The present study confirms reports that these haemoglobins are identical with the haemoglobins from the definitive erythrocytes found in developing chick embryos. Embryonic definitive erythrocytes contain a minor haemoglobin, H, in addition to haemoglobins A and D. Comparisons of the various α and β globin chains by tryptic peptide mapping, N-terminal amino acid sequence analyses, amino acid analyses and derivation of diference indices to assess compositional relatedness between the chains, implicate five structural genes in the synthesis of chicken haemoglobins A(α 2 A β 2 A ), D(α 2 D β 2 D ) and H(α 2 A β 2 H ). The globin chains β A , β D and β H are very similar in structure, whereas α A and α D differ markedly from each other and from the β chains.