Hemoglobin zeta(2)beta(2)(S) is generated by substituting embryonic zeta-globin subunits for the normal alpha-globin components of Hb S (alpha(2)beta(2)(S)). This novel hemoglobin has recently been shown to inhibit polymerization of Hb S in vitro and to normalize the pathological phenotype of mouse models of sickle cell disease in vivo. Despite its promise as a therapeutic tool in human disease, however, the basic O(2)-transport properties of Hb zeta(2)beta(2)(S) have not yet been described. Using human hemoglobins purified from complex transgenic-knockout mice, we show that Hb zeta(2)beta(2)(S) exhibits an O(2) affinity as well as a Hill coefficient, Bohr response, and allosteric properties in vitro that are suboptimally suited for physiological O(2) transport in vivo. These data are substantiated by in situ analyses demonstrating an increase in the O(2) affinity of intact erythrocytes from mice that express Hb zeta(2)beta(2)(S). Surprisingly, though, co-expression of Hb zeta(2)beta(2)(S) leads to a substantial improvement in the tissue oxygenation of mice that model sickle cell disease. These analyses suggest that, in the context of sickle cell disease, the beneficial antisickling effects of Hb zeta(2)beta(2)(S) outweigh its O(2)-transport liabilities. The potential structural bases for the antisickling properties of Hb zeta(2)beta(2)(S) are discussed in the context of these new observations.