Hydrogen peroxide is currently the most widely used apoptosis inducer due to its broad cytotoxic efficacy against nearly all cell types. However, equivalent cytotoxicity is achieved over a wide range of doses, although the reasons for this differential sensitivity are not always clear. In this study, three kinds of cells, the 293T cell line, primary fibroblasts, and terminally differentiated myocardial cells, were treated with a wide range of H2O2 doses. Times to apoptosis initiation and end were measured cytochemically and the changes in expression of caspase-9, P53, NF-κB, and RIP were determined by RT-PCR. The 293T cell line was the most sensitive to H2O2, undergoing necroptosis and/or apoptosis at all concentrations from 0.1 to 1.6 mM. At > 0.4 mM, H2O2 also caused necroptosis in primary cells. At < 0.4 mM, however, primary cells exhibited classic signs of apoptosis, although they tended to survive for 36 hours in < 0.2 mM H2O2. Thus, H2O2 is a broadly effective apoptosis inducer, but the dose range differs by cell type. For cell lines, a low dose is required and the exposure time must be reduced compared to primary cells to avoid cell death primarily by necroptosis or necrosis.