Sheep hemoglobin (SHb) was modified with methoxy-polyethylene glycol (mPEG) to develop a potential blood substitute. mPEG has been used to decrease antigenicity and immunogenicity of foreign proteins. When the mPEG was attached to SHb, the modified hemoglobins showed decreased electrophoretic mobility on SDS-PAGE and decreased free amino groups. When the remaining free amino groups of mPEG modified SHb were determined by TNBS free amino group titration methods. about 34% of total free amino groups were modified with mPEG. This mPEG-SHb conjugate of 34% amino groups modified showed no precipitation by double immunodiffusion with polyclonal antibodies against SHb. This modified hemoglobin still has oxygen transport activity. So this antigenicity decreased hemoglobin may be used in humans as a potential blood substitute.