Saikosaponin-d (SSd), a saponin derivative with a similar structure to estradiol, was extracted from Bupleurum falcatum L. (Umbelliferae). It was found that SSd stimulated the proliferation of MCF-7 cells by using MCF-7 cell proliferation assay. Cell-cycle analysis revealed that the proliferation-stimulating effect was associated with a marked increase in the number of MCF-7 cells in S phase. These actions of SSd were dose-dependent at doses ranging from 10nM to 10 microM and could be significantly inhibited by the specific estrogen receptor (ER) antagonist ICI-182780. Co-incubation of MCF-7 cells with 1 microM of ER antagonist ICI-182780 abolished the inductive effects of SSd on estrogen response element (ERE)-luciferase activity, suggesting that the estrogenic effects of SSd were mediated through the estrogen receptors. To evaluate the relative involvement of estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta) in mediating the actions of SSd, ER-negative human cervical carcinoma (HeLa) cells were cotransfected with the ERE-luciferase reporter construct and either ERalpha or ERbeta construct. The results showed that SSd could activate ERE-luciferase activity via the ERalpha-mediated pathway in a dose-dependent manner (10 nM to 10 microM); whereas, the activation of ERbeta-mediated ERE-luciferase activity by SSd only occurred at a high concentration (10 microM). Furthermore, the ERalpha protein and mRNA levels were increased by treatment with SSd within 24h. These data support our hypothesis that SSd acts as a weak phytoestrogen. Presumably, the estrogenic effect of SSd is mediated by the estrogen receptor.