Hb Geelong [β139(H17)Asn→Asp] was detected in a German woman of Polish-Russian descent. It is an unstable variant which appears to increase the severity of a (β+-thalassemic phenotype in the propositus. The electrophoretic properties of Hb A and Hb Geelong are similar on cellulose acetate in both acidic and alkaline conditions. The electrophoretic mobility and the amino acid analysis ofβxT-14 indicated the substitution Asn→Asp atβ139. The sequence ofβXT-14 was confirmed by dansyl-Edman degradation. The slight increase observed in the P50of whole blood is not intrinsic to theβ139 substitution, but is thought to result from an increased 2,3-diphosphoglycerate level in response to anemia. No family studies were possible to investigate the mode of inheritance of eitherβ+-thalassemia or Hb Geelong in the propositus. Synthetic globin chain ratios suggest that impaired synthesis of the variant globin chain is partially responsible for the low level of Hb Geelong in peripheral blood.