In vitro, resveratrol inhibited growth of 4T1 breast cancer cells in a dose- and time-dependent manner. In vivo, however, resveratrol had no effect on time to tumor take, tumor growth, or metastasis when administered intraperitoneally daily (1, 3, or 5 mg/kg) for 23 days starting at the time of tumor inoculation. Resveratrol had no effect on body weight, organ histology, or estrous cycling of the tumor-bearing mice. Resveratrol, therefore, is a potent inhibitor of 4T1 breast cancer cells in vitro; is nontoxic to mice at 1-5 mg/kg; and has no growth-inhibitory effect on 4T1 breast cancer in vivo.