The experiments described in this report used a well-characterised mouse breast cancer cell line to ask whether commercially available extracts of Actaea racemosa, a herb widely used by breast cancer patients, altered the response of cancer cells to radiation and to four drugs commonly used in cancer therapy. The A. racemosa extracts increased the cytotoxicity of doxorubicin and docetaxel, and decreased the cytotoxicity of cisplatin, but did not alter the effects of radiation or 4-hydroperoxycyclophosphamide, an analogue of cyclophosphamide that is active in cell culture. These data sound a warning that the herbal medicines being used by patients undergoing cancer therapy can have effects on cancer cells that alter their response to the agents commonly used to treat breast cancer.