In the interest of creating readily transfusable blood, we have developed a method of covalently bonding methoxy polyethylene gylcol (mPEG) to the erythrocyte membrane to mask antigens which would otherwise cause immunorejection in unmatched blood transfusions. Previous studies have reported the ability of mPEG to mask erythrocyte antigens without significantly affecting viability, morphology, deformability, and other membrane functions. Our spectrophotometric analysis of modified erythrocytes has shown that as well as its other properties, mPEG-coated red blood cells can still take up oxygen, while remaining antigenically silent.