(A) Changes in pH, temperature, and derivatization time did not effect the ability of human RBC to be effectively modified with mPEG (1.2 mmol/L), as shown by comparable inhibition of antiserum-mediated aggregation. (B) Furthermore, addition of mPEG-modified (2.4 mmol/L) RBC to control cells diminished antisera-mediated aggregation in a dose-dependent manner. The values shown are the percentage of mPEG-modified RBC. The above figures are representative of more than 5 independent assays.