Hemoglobin M Iwate is a mutant in which the proximal histidine residue (F8) of theαchain is replaced by tyrosine. One of consequences of this mutation is the oxidation of the iron atom of theαchain, while theβchain of the tetramer remains in the ferrous state, capable of binding oxygen. NMR experiments (1) and X ray analysis (2) have shown that the quaternary structure of this mutant hemoglobin is almost frozen in the T state, no matter whether its normalβhemes are unliganded or liganded, in solution as well as in the crystal. Baldwin has estimated the allosteric equilibrium constant for Hb M Iwate in phosphate buffer as about 105in favor of the T state (3). It was of interest, therefore, to see the effect of organic phosphate on the oxygen equilibrium function of ferrousβhemes of the mutant.