Study summary: The present study was designed to examine whether certain herbs alter proliferation of breast cancer cells that are estrogen receptor (ER) positive but differ in the expression of ER and ERß, and whether these herbs also alter cellular response to estradiol (E2). The breast cancer cell lines MCF-7 (60:40 ER:ERß) and T47D (33:67 ER:ERß) were treated with various strength dilutions of standard extracts of angelica (dang gui), reishi mushroom (ling zhi), licorice (gang cao), fo-ti (he show wu), and astragalus (huang qi) in the presence or absence of estradiol (1nm). Two controls were used: E2 treatment only and no treatment. The proliferation index was determined after 72 hours and values for the herbal treatments were compared to those of the controls. The MCF-7 cells were found to respond more strongly to E2 than did the T47D cells. The most noteworthy difference between these two cell lines was seen in their responses to fo-ti, reishi mushroom, and angelica. The MCF-7 cells either showed no response (fo-ti) or only a slight response (angelica and reishi mushroom) to the herbal extracts. On the other hand, the T47D cells exhibited a dose-related upswing in proliferation at higher doses of fo-ti, reishi mushroom and angelica, with the response to angelica and reishi mushroom exceeding the E2 control levels. This differential response was also found when both cancer cell lines were treated with these three herbs along with E2. In the MCF-7 cells, E2 tended to mask the effects, if any, of these herbs. In T47D cells, however, the addition of E2 to both reishi mushroom and angelica resulted in an additive effect, the proliferation index exceeding E2-stimulated levels at the higher doses. Fo-ti combined with E2 in the T47D cells was found to have no effect on E2-induced proliferation. Astragalus without E2 was found to be biphasic in both cell lines, with proliferative effects at the highest doses reaching E2 control levels in MCF-7 cells and exceeding E2 control values in T47D cells. Astragalus combined with E2 had no effect on the E2-induced proliferation in MCF-7. However, in T47D cells, astragalus plus E2 maintained a biphasic response with enhancement of proliferation at higher doses. Licorice alone showed a proliferative effect equivalent to that of the E2 control in MCF-7 cells; in T47D cells, licorice produced a strong dose response exceeding that of the E2 control. These responses did not change when E2 was added to licorice in both cell lines. These results show that some traditional Chinese medicine herbs used for immune system support may be estrogenic, and that the response differs depending on the breast cancer cell line and the presence or absence of estradiol. Generally speaking, T47D cells respond most strongly to these herbs, most likely because of their higher content of ERß, which has a higher affinity than ER for phytoestrogens. The authors conclude that the estrogenicity of these herbal preparations should be taken into account by those who want to reduce their estrogen exposure.