No differences are observed in T-cell proliferation when control and mPEG-modified PBMC are exposed to the mitogen PHA. Cell proliferation is measured by ³H-thymidine incorporation into the DNA of PBMC (2.5 × 10⁵ PBMC) in response to PHA stimulation for the duration of the 4-day culture (see Materials and Methods). The results shown are the mean ± SD of triplicate samples from a representative experiment of 4 independent studies. Insert: Photomicrographs of mPEG-derivatized lymphocytes (1.2 mmol/L mPEG per 4 × 10⁶ PBMC) without PHA treatment (A) and with PHA treatment (B) after 4 days of culture.