The in vivo circulation of hyperbranched polyglycerol (HPG) grafted red blood cells (RBCs) was investigated in mice. The number of HPG molecules grafted per RBC was measured using tritium labeled HPGs ((3)H-HPG) of different molecular weights; the values ranged from 1 × 10(5) to 2 × 10(6) molecules per RBC. HPG-grafted RBCs were characterized in vitro by measuring the electrophoretic mobility, complement mediated lysis, and osmotic fragility. Our results show that RBCs grafted with 1.5 × 10(5) HPG molecules per RBC having molecular weights 20 and 60 kDa have similar characteristics as that of control RBCs. The in vivo circulation of HPG-grafted RBCs was measured by a tail vain injection of (3)H-HPG60K-RBC in mice. The radioactivity of isolated RBCs, whole blood, plasma, different organs, urine and feces was evaluated at different time intervals. The portion of (3)H-HPG60K-RBC that survived the first day in mice (52%) remained in circulation for 50 days. Minimal accumulation radioactivity in organs other than liver and spleen was observed suggesting the normal clearance mechanism of modified RBCs. Animals gained normal weights and no abnormalities observed in necropsy analysis. The stability of the ester-amide linker between the RBC and HPG was evaluated by comparing the clearance rate of (3)H-HPG60K-RBC and PKH-26 lipid fluorescent membrane marker labeled HPG60K-RBCs. HPG modified RBCs combine the many advantages of a dendritic polymer and RBCs, and hold great promise in systemic drug delivery and other applications of functional RBC.