In the hemoglobin of the opossum, the alpha chains have different residues at positions E7 and E11 than do most other mammalian hemoglobins. In the opossum, the usual histidine at alpha E7 is replaced by glutamine, the valine at alpha E11 by isoleucine, and the hemoglobin is known to have a low oxygen affinity and a low Hill coefficient. Comparison of kinetic studies of opossum hemoglobin with normal human hemoglobin shows that alpha chains in Hb opossum, despite the lack of distal histidine, do not differ significantly in CO-combination rates in either the T or R states. These rates are much slower than the rates reported for Hb Zurich, the hemoglobin from Chironomus thumi thumi, or the monomeric components of glycera hemoglobin, all of which also have a different residue at E7. As compared with Hb A, the changes in ligand affinities in the T and R states are small and cannot account for the unusually high values of p50 for Hb opossum. The equilibrium and kinetic data indicate that the L = (T)/(R) is about 100 times higher for Hb opossum than for Hb A; CCO = KR/KT approximately equal to 0.014. The kinetic data on l'4 and l also indicate that the R leads to T equilibrium for Hb4(CO)4 and Hb4(CO)2 can be shifted in either direction by adding inositol hexaphosphate or by changing the pH.