Hemoglobin North Chicago,β36 [C2] Pro→Ser is a new high oxygen affinity hemoglobin variant. It was discovered in a 52-year-old male with erythrocytosis since age 20 who had been treated with different regimens for polycythemia vera including several courses of32P. The variant is electrophoretically silent with normal stability and increased oxygen affinity (P5016.6 mm Hg at 37°C., pH 7.4). Characterization of the structure of hemoglobin North Chicago involved the use of HPLC, secondary ion mass spectral analysis of the tryptic peptides and conventional fingerprinting. Hemoglobin North Chicago manifested bizarre hydrophobicity of itsβ-chains, as demonstrated by reverse phase HPLC and Triton X-100 electrophoresis. This behavior is not expected from the substitution of proline to serine. Proline residueβ36 [C2] is one of the invariant residues of theβ-chains of all known mammals and most vertebrates. This residue is involved in theα1β2contacts of hemoglobin molecule and its substitution to serine is possibly associated with conformational changes and alteration of hemoglobin function.