Retroviral transduced P-gp^+ve cells are resistant to TNF-mediated, caspase-dependent DNA fragmentation and membrane lysis. Parental 12D7 (P-gp^−ve) and retrovirus-transduced 12D7-MDR1 (P-gp^+ve) cells were labeled with ¹²⁵IUdR (A) and ⁵¹Cr (B) for 1 hour, washed in growth media, and incubated for 48 hours in 96-well plates (2 × 10⁴ cells/well) with recombinant soluble TNF (0.2 and 2.0 μg/mL). In some wells, cells were preincubated for 30 minutes with 20 μmol/L ZVAD-fmk or control ZFA-fmk inhibitor and/or anti–P-gp MoAb MRK 16 (50 μg/mL) as indicated in Table 1. Data are calculated as the mean ± SE of triplicate samples and are representative of at least two different experiments.