The disclosure relates to recombinant adult human apohemoglobin (apo-rHb) in which the stability has been increased by replacement of an amino acid with a counterpart from another organism, such as a deep sea diving mammal. This mutated apo-rHb may be more stable and/or give higher production yields than unmutated adult human apo-rHb. The mutated apo-rHb may be produced in microorganisms, such as E. coli or yeast cells, or animal erythroid cells. Some apo-rHb of the present disclosure may be used as part of a blood substitute.