Sickle hemoglobin (Fe(II)HbS) reconstituted with nickel(II)protoporphyrin IX yields an artificial hemoglobin (Ni(II)HbS), the first heme-substituted hemoglobin shown to mimic the polymerization of deoxyHbS. Unlike Fe(II)HbS, Ni(II)HbS does not bind oxygen and therefore polymerizes under aerobic conditions. While the polymer solubility coefficient (Csat) for Ni(II)HbS is elevated about 2 g/100 mL compared with that for deoxy Fe(II)HbS, hemoglobin concentration in the polymer phase is the same. Electron micrographs of thin sections of embedded Ni(II)HbS reveal 20-nm-diameter fibers indistinguishable from those seen with deoxygenated native HbS. Nickel(II)HbS can be used in studies on the sickling process and on antisickling agents that could not previously be done or were difficult to execute because of the need for an anaerobic environment.