A new hyperunstable hemoglobin was found in a Japanese girl who had very severe, chronic hemolytic anemia. Her parents and siblings were hematologically normal. The abnormal hemoglobin comprised a very small proportion of the total hemoglobin, although it was produced almost at the same rate as normal hemoglobin. Sequencing of an abnormal peptide which was liberated from the beta chain by hydrolysis with a protease from Staphylococcus aureus V8 disclosed the tandem insertion of a five-residue segment which included the proximal histidine at beta 92(F8).