A high oxygen affinity hemoglobin, previously undescribed, was found in a healthy, asymptomatic patient with mild erythrocytesis and left-shifted hemoglobin-O₂ dissociation curve. The hemoglobin variant could not be distinguished from Hb A by any of several electrophoretic methods nor by ion exchange chromatography. It was separated and analyzed by reversed phase high performance liquid chromatography. Structural analysis revealed the substitution β109 (Gil) Val-Leu. The variant was named Hb Johnstown. The amino acid substitution perhaps disrupts α,β, contacts in the deoxyhemoglobin conformation, thus shifting the equilibrium towards the high affinity oxyhemoglobin conformation.