Each year the United States population receives an estimated 12 to 14 million units of packed red blood cells (RBCs) and whole blood. 1,2 It is estimated that 33% of transfusions associated with trauma are with unmatched type O RBCs (UORBC). 3 UORBCs have been proven effective and relatively safe 3 however, by masking RBC surface antigens the risk of transfusion reaction may be further decreased. It is, therefore, important to evaluate and validate the stability of antigen masked RBCs, which may play a part in avoiding transfusion reactions. These antigen-masked RBCs would be regularly subjected to abnormal in vivo conditions commonly associated with massive transfusion such as lactic acidosis, bacteremia, and in vitro irradiation, which is frequently used to sterilize and decrease T Lymphocyte counts in RBC units before transfusion. This study compared two methods of masking RBC antigens by PEGylation: maleimide-PEGylation and cyanuric chloride-PEGylation. RBC PEGylation effectively masks the Rh(D) antigen 4,5 and PEG-RBC bond stability was evaluated by comparison of pre and post exposure agglutination with anti-D sera. While the stability of maleimide-PEG-RBCs remained unaffected, the cyanuric chloride-PEG-RBCs remained stable in the bacteremia and irradiation studies, but critical concentrations of lactic acid caused dePEGylation. Further studies are warranted to ensure in vivo stability.