An abnormal hemoglobin variant was detected in a seven-year old Thai girl. The clinical and hematological pictures were indistinguishable from hemoglobin H disease. Upon cellulose acetate electrophoresis at pH 8.5 an abnormal hemoglobin variant was found together with hemoglobins A and H. The variant moved slightly behind HbF and amounted to approximately 9%. Globin chain synthesis studies of peripheral blood reticulocytes incubated with tritiated leucine revealed that the total radioactivity ratio α-chain + variant a-chain/β-chain was 0.59, while the ratio variant α-chain/β-chain was 0.12. The variant hemoglobin was somewhat unstable under heat denaturation and in the isopropanol test. Structural studies by component isolation, globin chain separation, peptide mapping and aminoacid analysis of abnormal peptides showed that the leucine residue 109 of the α-chain was replaced by arginine.