We studied the mechanism of the cytotoxic activity of BZL101, an aqueous extract from the herb Scutellaria barbata D. Don, which is currently in phase II clinical trial in patients with advanced breast cancer. The phase I trial showed favorable toxicity profile and promising efficacy. We report here that BZL101 induces cell death in breast cancer cells but not in non-transformed mammary epithelial cells. This selective cytotoxicity is based on strong induction by BZL101 of reactive oxygen species (ROS) in tumor cells. As a consequence, BZL101 treated cancer cells develop extensive oxidative DNA damage and succumb to necrotic death. Data from the expression profiling of cells treated with BZL101 are strongly supportive of a death pathway that involves oxidative stress, DNA damage and activation of death-promoting genes. In breast cancer cells oxidative damage induced by BZL101 leads to the hyperactivation of poly (ADP-ribose) polymerase (PARP), followed by a sustained decrease in levels of NAD and depletion of ATP, neither of which are observed in non-transformed cells. The hyperactivation of PARP is instrumental in the necrotic death program induced by BZL101, because inhibition of PARP results in suppression of necrosis and activation of the apoptotic death program. BZL101 treatment leads to the inhibition of glycolysis selectively in tumor cells, evident from the decrease in the enzymatic activities within the glycolytic pathway and the inhibition of lactate production. Because tumor cells frequently rely on glycolysis for energy production, the observed inhibition of glycolysis is likely a key factor in the energetic collapse and necrotic death that occurs selectively in breast cancer cells. The promising selectivity of BZL101 towards cancer cells is based on metabolic differences between highly glycolytic tumor cells and normal cells.