A hitherto undescribed, unstable hemoglobin was discovered in a four-year-old Japanese girl with relatively severe hemolytic anemia requiring monthly blood transfusion Although no abnormal hemoglobin was detectable by electrophoresis at pll 8.6 and 7.0, heat denaturation and isopropanol tests gave positive results.Chemical analyses of the heat labile henioglohin have demonstrated an amino acid substitution of a prolyl for the leucyl residue at the 68th (E 12) position.Remarkable clinical improvement was observed after splenectomy.