Human hemoglobin binds oxygen cooperatively and functions as a tetramer composed of two identical alphabeta heterodimers. While human hemoglobin is the best characterized allosteric protein, the quaternary R (oxygenated or liganded) to T (deoxygenated) structural transition remains controversial. The R2 state has been postulated to represent either an intermediate or final quaternary state elicited by ligand binding. However, the biological relevance of the R2 state has been questioned as it has not been observed crystallographically under physiological conditions. The high-resolution R2 quaternary structures of human COHbC (betaE6K) and COHbS (betaE6V) are reported at neutral pH and low ionic strength using PEG 4000 as a precipitant. Crystals of COHbC, COHbS and their mixtures are isomorphous, indicating that they share the same tertiary and quaternary structures. In contrast, oxyHbA or COHbA did not yield crystals at neutral pH under similar conditions. Solubility studies and modeling suggest that at neutral pH and low ionic strength the beta6 mutant hemoglobins crystallize (betaK6 > betaV6) as a result of more favorable lattice contacts.