Hb M Akita disease is an example of Hb M Hyde Park hemoglobinopathy found in Japan. It causes cyanosis and mild hemolytic anemia. A minor slow-moving red component of hemoglobin is always detected electro-phoretically together with the main chocolate-brown component of abnormal hemoglobin (Hb M Hyde Park = Hb M Akita,β92 HisàTyr). The minor component has been called Hb Akita by us since its discovery, but Hb Hyde Park or Hb HP will be the preferred name.Purified solutions of Hb HP were prepared by cellulose acetate membrane electrophoresis (pH 8.6), and its physicochemical properties were investigated in comparison with those of Hb A and Hb M Hyde Park by spectroscopy, observation of ligand reactivity and oxygen affinity (Bohr effect and the 2,3-DPG effect), starch gel electrophoresis of PCMB-treated globins, and fingerprinting of the trypsin digest of aminoethylated polypeptide chains separated by urea CMC chromatography.The results obtained revealed that (1) Hb HP possessedtheabnormalβMchain which was the same as that of Hb M Hyde Park, and (2) in the molecule of lib HP one of theβMchain pair was without heme, and (3) all of the heme irons of the normalαchains (αA) and the heme-retaining abnormalβMchain were ferrous. Accordingly, Hb HP is presumed to be a modified hemoglobin expressed by the formulaα2A(2h)βM(1h)βM(oh), where 2h, 1h and oh refer to the presence of two, one and zero hemes, respectively.