Cytotoxicity is catalase-dependent. Cell number was determined by Coulter counting following 24-hour and 48-hour exposures to 50 μM MGd and 50μM ascorbate (○) in the dexamethasone-sensitive cell line, C2E3, and the dexamethasone-resistant line, 1-310. Catalase (260 U/mL) was added (^*) at time 0 to C2E3 cells (A) and 1-310 cells (B). Exposure to 100 μM Asc alone is indicated by □; to 50 μ MGd alone, ▵. Mannitol control is indicated by ⋄. Cell viability by PI assessment of membrane permeability was measured following 18-hour and 50-hour exposures to 50 μM MGd in C2E3 (C), and 24-hour and 48-hour exposures to 50 μM MGd in 1-310 (D). Catalase blocks loss of cell viability induced by MGd and ascorbate as measured by PI. Results shown (means and SD) were obtained from triplicate measurements at each time point. Controls included mannitol, 50μM MGd, 100 μM ascorbate, and combined catalase/ascorbate (data not shown for the latter).