Correlation of (A) apoptosis, (B) PS externalization, and (C) ICE family protease activation determined by actin cleavage assay and (D) CPP32 processing in U937 and its apoptosis-resistant variant cell lines, UK711 and UK110. Cells were treated without (C) or with 10 μg/mL VP16 (V) or 150 ng/mL anti-Fas antibody and 1 μg/mL cycloheximide (F ) for 4 hours. Numbers in parentheses in (A) and (B) are the percentages of apoptotic cells and PS externalized cells, respectively. The experiments were repeated four times and similar results were obtained.