In this study, we investigated the importance of redox and Bcl-2 status on cytochrome c-mediated apoptosis. Two mouse lymphoma cell lines, LYas and LYar that express Bcl-2 protein at different levels, were used to reconstitute a cell-free system. Cytoplasmic extracts made from apoptosis-sensitive LYas cells 2.5 h after exposure to 5 Gy gamma-radiation were able to induce apoptosis in isolated nuclei, whereas extracts made from LYas cells at time points earlier than 2. 5 h, or from Bcl-2-overexpressing, apoptosis-resistant LYar cells at all time points after irradiation were inactive. Apoptotic activity was restored to inactive extracts by the addition of oxidized but not reduced cytochrome c. Cytochrome c reductase was able to inhibit apoptosis in extracts made from LYas cells 2.5 h after irradiation and LYar extracts activated by addition of oxidized cytochrome c. Antioxidants, but not oxidant defensive enzymes, blocked apoptosis implying that antioxidants might alter the redox state of factors important in mediating apoptosis. These findings confirm the importance of cellular redox state during apoptosis and are consistent with a role for Bcl-2 in regulating this redox state.