One of the main difficulties with blood substitutes based on hemoglobin (Hb) solutions is the auto-oxidation of the hemes, a problem aggravated by the dimerization of Hb tetramers. We have employed a method to study the oxyHb tetramer-dimer equilibrium based on the rate of auto-oxidation as a function of protein concentration. The 16-fold difference in dimer and tetramer auto-oxidation rates (in 20 mM phosphate buffer at pH 7.0, 37 "C) was exploited to determine the fraction dimer. The results show a transition of the auto-oxidation rate from low to high protein concentrations, allowing the determination of the tetramer-dimer dissociation coefficient14-fold increase in K4,2 was observed for addition of 10 mM of the allosteric effector inositol hexaphosphate (IHP).Recombinant hemoglobins (rHb) were genetically engineered to obtain Hb with a lower oxygen affinity than native Hb (Hb A). The rHb a 9 2 [(C7) F41Y/(G4) N102Yl shows a fivefold increase in K4.2 at pH 7.0,37"C. An atmosphere of pure oxygen is necessary in this case to insure fully oxygenated Hb. When this condition is satisfied, this method provides an efficient technique to characterize both the tetramer-dimer equilibrium and the auto-oxidation rates of various oxyHb. For low oxygen affhity Hb equilibrated under air, the presence of deoxy subunits accelerates the auto-oxidation. Although a full analysis is complicated, the auto-oxidation studies for air equilibrated samples are more relevant to the development of a blood substitute based on Hb solutions. The double mutants, rHb a2p2 [(C7) F41Y/(G4) N102Al and rHb a2p2 [(C7) F41Y/(E10) K66T], show a lower oxygen afflnity and a higher rate of oxidation than Hb A. Simulations of the auto-oxidation rate versus Hb concentration indicate that very high protein concentrations are required to observe the tetramer auto-oxidation rate. Because the dimers oxidize much more rapidly, even a small fraction dimer will influence the observed oxidation rate.Keywords: auto-oxidation; hemoglobin; oxyhemoglobin; recombinant hemoglobin; tetramer-dimer equilibrium Despite the vast amount of research on hemoglobin (Hb), there are no rapid and reliable methods to measure the fraction of dimers. This is due to the difficulty of obtaining a reliable signal that differenciates between the dimeric and tetrameric species. Dimers have spectral and ligand binding properties similar to fully liganded tetramers. In addition, it is difficult to obtain data for pure dimers at pH 7, because one must use protein concentrations below 100 nM-a condition where the equilibria for the globin-heme (or hemin) and the monomerization may influence the results.