We have engineered a stable octameric hemoglobin (Hb) of molecular mass 129 kDa, a dimer of recombinant hemoglobin (rHbbG83C-F41Y) tetra-mers joined by disulfide bonds at theb83 position. One of the major prob-lems with oxygen carriers based on acellular hemoglobin solutions is vasoactivity, a limitation which may be overcome by increasing the mole-cular size of the carrier.