The structure, properties and function of, and some bio-synthetic and genetic aspects of, Hb A2are described. The structural variants of the A2are reviewed and their geographical distribution presented. Hb A2', Hb A2-Flatbush and Hb A2-Babinga are characteristic of negro populations and may have originated in Western or Central Africa. Hb A2-Sphakia is characteristic of Canadian Amerindian and lib A2-Indonesia of Indonesian/Malay populations. Hb A2-NYU has only been found sporadically and most frequently in persons of Eastern European origin. The other three variants of Hb A2have only been reported in a single person or in single families. Some conditions which are associated with changes in Hb A2levels are reviewed.