An electrophoretically slowly moving hemoglobin variant, Hb Yusa, was found in a 48-year-old man, his younger sister and mother. No clinical or hematological abnormalities were seen in the individuals heterozygous for this mutant gene. Structural studies of this abnormal hemoglobin showed a new type substitution of beta 21 (B3) Asp leads to Tyr. The percentage of the abnormal hemoglobin in the hemolysate was 40.8. The hemoglobin had neither instability nor functional abnormality.