This paper describes a method for isolating high oxygen affinity hemoglobins. It involves the selective derivatisation of the cysteine residue at. position 93 in theβchain. The iodoacetamide derivative of the high oxypn affinity hemoglobin was separated from the more negatively charged iodoacetic acid derivative of HbA on DEAE-Sephadex. The method was used to isolatp two high oxygen affinlty hemoglobins,oneof which was subseqrtmtly identified as llb Heathrow.