Hemoglobin M Hyde Park (Hb M H) and its modified product (Hb M HX) in oxygenated form were obtained by Chromatographic procedures. The patient's blood containing Hb M H revealed EPR spectra with both high and low spin-type signals. In an O 2 equilibrium study, Hb M H showed similar functional abnormalities to Hb M Saskatoon, a somewhat higher O 2 affinity and a smaller n value than Hb A, and a Bohr effect that was almost normal. But the n value and the O 2 affinity of the O 2 equilibrium curves varied considerably according to the wavelength used for calculation, suggesting the conformational change of the abnormal subunit of Hb M H. The abnormal subunit of methemoglobin M H was able to react with cyanide, and its K′ value was similar to that of Hb M Boston. Heat-denaturation also disclosed the remarkable instability of Hb M H, which was compatible with the results of the in vivo hematologic study.