The purpose of this report is to explore the growth inhibitory effect of extracts and compounds from black cohosh and related Cimicifuga species on human breast cancer cells and to determine the nature of the active components. Black cohosh fractions enriched for triterpene glycosides and purified components from black cohosh and related Asian species were tested for growth inhibition of the ER(-) Her2 overexpressing human breast cancer cell line MDA-MB-453. Growth inhibitory activity was assayed using the Coulter Counter, MTT and colony formation assays. Results suggested that the growth inhibitory activity of black cohosh extracts appears to be related to their triterpene glycoside composition. The most potent Cimicifuga component tested was 25-acetyl-7,8-didehydrocimigenol 3-O-beta-d-xylopyranoside, which has an acetyl group at position C-25. It had an IC(50) of 3.2microg/ml (5microM) compared to 7.2microg/ml (12.1microM) for the parent compound 7,8-didehydrocimigenol 3-O-beta-d-xylopyranoside. Thus, the acetyl group at position C-25 enhances growth inhibitory activity. The purified triterpene glycoside actein (beta-d-xylopyranoside), with an IC(50) equal to 5.7microg/ml (8.4microM), exhibited activity comparable to cimigenol 3-O-beta-d-xyloside. MCF7 (ER(+)Her2 low) cells transfected for Her2 are more sensitive than the parental MCF7 cells to the growth inhibitory effects of actein from black cohosh, indicating that Her2 plays a role in the action of actein. The effect of actein on Her2 overexpressing MDA-MB-453 and MCF7 (ER(+)Her2 low) human breast cancer cells was examined by fluorescent microscopy. Treatment with actein altered the distribution of actin filaments and induced apoptosis in these cells. These findings, coupled with our previous evidence that treatment with the triterpene glycoside actein induced a stress response and apoptosis in human breast cancer cells, suggest that compounds from Cimicifuga species may be useful in the prevention and treatment of human breast cancer.