Hemoglobin Cubujuqui (1) was detected in several members of a Mexican-American family. Structural analysis of this hemoglobin indicated that the carboxyl terminal arginine at position 141 in theαchain had been replaced by serine. This residue is critical not only in stabilizing the deoxy or T conformation by electrostatic interactions, but it is also involved in the Bohr effect through its linkage with Val lαof the opposite a chain in the tetramer. The variant exhibits high affinity for oxygen that is associated with destabilization of the deoxy conformation, and reduced cooperativlty. The pH and the chloride sensitivity of the variant are also reduced, as compared to Hb A.