A new hemoglobin variant, hemoglobin (Hb) Nunobiki, was detected in a Japanese male with marginal erythrocytosis. The Hb Nunobiki component amounted to 13.1% of the total hemoglobin. Structural analysis of this variant established the substitution of a cysteine for an arginine at the carboxy terminus of the alpha-chain (alpha 141). The oxygen equilibrium curves of Hb Nunobiki revealed extremely high oxygen affinity with a reduced Hill coefficient n, a decreased alkaline Bohr effect, and a decreased 2,3-diphosphoglyceric acid effect. The isoelectric point of the Hb Nunobiki changed during storage, although the oxyhemoglobin state was maintained. These findings could be accounted for by the specific characteristics of a newly introduced cysteinyl residue. Cysteinyl residue at alpha 141 in Hb Nunobiki did not seem to be involved in the formation of either intermolecular or intramolecular disulfide bonds under physiologic conditions. The low proportion of Hb Nunobiki (13.1%) in the propositus was also discussed after it was verified that he exhibited four alpha-globin genes per diploid cell.