The effect of Fas antagonist ZB4 and Fas agonist CH11 antibodies on ara-G–mediated cell death. Exponentially growing cultures of CEM (upper panels), Raji (middle panels), and ML-1 (lower panels) were either untreated or treated with ZB4, the Fas antagonist antibody; ara-G; ZB4 for 1 hour followed by ara-G; or CH-11, the Fas agonist antibody. Cells were analyzed by flow cytometry for TUNEL positivity and their cell cycle profile.