Artemisinin becomes cytotoxic in the presence of ferrous iron. Since iron influx is high in cancer cells, artemisinin and its analogs selectively kill cancer cells under conditions that increase intracellular iron concentrations. We report here that after incubation with holotransferrin, which increases the concentration of ferrous iron in cancer cells, dihydroartemisinin, an analog of artemisinin, effectively killed a type of radiation-resistant human breast cancer cell in vitro. The same treatment had considerably less effect on normal human breast cells. Since it is relatively easy to increase the iron content inside cancer cells in vivo, administration of artemisinin-like drugs and intracellular iron-enhancing compounds may be a simple, effective, and economical treatment for cancer.