An electrophoretically fast-moving hemoglobin variant was found to be present together with hemoglobin S, in the hemolysate of the rythrocytes of at 3-yr-old Greek boy. Electrophoresis of the parents' erythrocyte hemolysates revealed that the father was an AS heterozygote, while the mother was a carrier of the variant hemoglobin. A sibling was also found to be a carrier. The amount of the mutant hemoglobin in the peripheral blood of the propositus, his mother, and his brother was 62.2%, 52.5%, and 51.1%, respectively, as determined by column chromatography. The patients peripheral blood smear showed mild anisocytosis, microcytosis, and hypochromia. Similar but less pronounced red cell abnormalities were found in the other two carriers. Structural analysis of the variant hemoglobin revealed substitution of an aspartic acid for the glycine residue at the beta83 (EF7) position. This new hemoglobin was named hemoglobin Pyrgos. All the carriers of hemoglobin Pyrgos are clinically healthy, and there seems to be no interaction between hemoglobin Pyrgos and hemoglobin S as manifested clinically.