BACKGROUND Membrane grafting of methoxypolyethylene glycol (mPEG) provides a unique strategy in preventing the immunologic recognition in blood transfusion. mPEG-modified red blood cells (mPEG-RBCs) have acceptable in vitro properties and provide a useful solution to problems with clinical blood matching. The aim of this study was to demonstrate the physiologic normality of mPEG-RBCs in mice. METHODS Mouse RBCs were withdrawn via cardiac bleed and modified with 1.0 mmol per L mPEG with succinimidyl propionate linker. The fluorescein-labeled mPEG-RBCs were then transfused into recipient mice for in vivo survival analysis. At the same time, the exsanguine mouse model was produced, and mice were transfused with mPEG-RBCs. The effects of mPEG-RBC transfusion on the hemoglobin (Hb) level, swimming endurance capacity, and hypoxic-ischemic injury in hippocampal pyramidal cells of exsanguine mice were investigated. RESULTS mPEG-RBCs showed the same in vivo survival curve and t((1/2)) as those of untreated RBCs. Transfusion of mPEG-RBCs could elevate Hb level of exsanguine mice and improve their swimming endurance capacity, and histologic studies showed that mPEG-RBCs could also restore the hypoxic-ischemic injury of hippocampal pyramidal cells in exsanguine mice, which were similar with control RBCs. That is, mPEG-RBCs functioned in a similar fashion to untreated RBCs in exsanguine mice. Therefore, these results revealed that mPEG-RBCs had normal oxygen-carrying capacity. CONCLUSIONS In conclusion, the results confirmed that mPEG-RBCs could perform their in vivo function of carrying O(2) and improve some physiologic indexes of exsanguine mice, and the physiologic normality of mPEG-RBCs provides new findings for clinical use.