Effects of catalase and H₂O₂ on neutrophil apoptosis. Neutrophils were cultured for 24 hours in the presence or absence of catalase and H₂O₂. Apoptosis was evaluated as described in Fig 1. To avoid inactivation of H₂O₂ by serum, serum-free medium was used to assess the effect of H₂O₂ on neutrophil apoptosis. This probably reflects increased apoptosis in H₂O₂-untreated control cells (70.8% ± 2.0%) compared with apoptosis in catalase-untreated control cells (62.8% ± 1.7%). Data represents mean ± SEM of 4 experiments. * P < .05, ** P < .01 versus control cells in the absence of catalase or H₂O₂.