Proliferation of the mPEG-modified PBMC is not affected by the addition of exogenous IL-2, suggesting that these cells are not receiving the initial, antigen recognition signals necessary to evoke a proliferation response. Cell proliferation is measured by³H-thymidine incorporation into the DNA of responder cells (2.5 × 10⁵ PBMC) in response to irradiated stimulator cells (2.5 × 10⁵ PBMC) and 0.83 U/mL IL-2 present for the duration of the 4-day culture. The results shown are the mean ± SD of triplicate samples from a representative experiment of 4 independent studies using pegylated responder cells. All values for the mPEG-derivatized cells differ from control PBMC at P < .0001.