The in vivo survival of mPEG-modified murine RBC was unaffected by derivatization at low concentrations of mPEG. Shown are the clearance rates of primary IP infusions of control and mPEG-modified (0.2 to 0.8 mmol/L) RBC. Survival was observed using a fluorescent fatty acid label (PKH-26), as previously described.4 5 Each transfusion approximated 8% to 10% of total mouse RBC mass. The results shown are the mean ± standard deviation of six Balb/c mice per mPEG condition, and the clearance slopes were determined by linear regression. (INSERT) Hypertransfusion of mice with autologous mPEG-modified RBC had no effect on mouse viability. Multiple transfusions of Balb/c mice (n = 6 per group) with mPEG-modified (0.4 mmol/L) or control RBC resulted in no apparent differences between groups in terms of viability or behavioral and physical activity. These data suggest a lack of toxicity and emphasize the in vivo normality of mPEG-modified RBC. The mice received a total of 33 transfusions (400 μL each of a 40% hematocrit) of control or mPEG-modified RBC.