A black 25-year-old woman and her father have a fast-moving alpha chain variant in an amount of 8% (the father) and 18% (the daughter). Structural data indicate that this chain has been elongated by the addition of three amino-acid residues to give the sequence: -Pro(114)-Ala(115)-Glu(116)-Phe(117)-Thr(118)-Glu-Phe-Thr-Pro(119)-Ala(120)-. The underlying genetic alteration responsible for hemoglobin Grady appears, therefore, to be a tandem duplication of nine base pairs which may have arisen by a process of mismatched intragenic crossing over. Functional and physicochemical properties of the variant are not greatly altered, and hematological data are normal.