RBC were readily modifiable by activated mPEG after extended periods of storage. However, a slight decrease in the inhibition of antisera-mediated aggregation was observed over the 45 days of storage. Shown are the aggregation curves of RBC following 0 (A), 7 (B), and 45 days (C) of storage in ACD at 4°C. RBC were derivatized with 0 to 1.2 mmol/L cyanuric chloride–activated mPEG at 4°C for 30 minutes in PBS (pH 9.2).