A newly developed recombinant yeast strain, in which the human estrogen receptor has been stably integrated into the genome of the yeast, was used to gain information on the estrogenic activity of a large series of dietary flavonoids. Among 23 flavonoids investigated, 8 were found to markedly stimulate the transcriptional activity of the human estrogen receptor in the yeast assay increasing transcriptional activity 5-13-fold above background level, corresponding to EC50 values between 0.1 and 25 mu M, Five compounds increased the transcriptional activity 2-5-fold over the control, with EC50 values ranging from 84 to 102 mu M, whereas the remaining flavonoids were devoid of activity. The most potent flavonoid estrogens tested were naringenin, apigenin, kaempferol, phloretin, and the four isoflavonoids equol, genistein, daidzein, and biochanin A. With the exception of biochanin A, the main feature required to confer estrogenicity was the presence of a single hydroxyl group in the 4'-position of the B-ring of the flavan nucleus, corresponding to the 4-position on phloretin. The estrogenic potency of the flavonoids was found to be 4 000-4 000 000 times lower than that observed for 17 beta-estradiol, when compared on the basis of EC50 values. The estrogenic activity of the dietary flavonoids was further investigated in estrogen-dependent human MCF7 breast cancer cells. In this system several of the flavonoids were likewise capable of mimicking natural estrogens and thereby induce cell proliferation. Similar structural requirements for estrogenic activity were found for the two assays. The present results provide evidence that several of the flavo-estrogens possess estrogenic properties comparable in activity to the well-established isoflavonoid estrogens. The use of Alamar Blue, a vital dye which is metabolically reduced by cellular enzymes to a fluorescent product, was found to greatly simplify the MCF7 cell-based estrogen screen, making this mammalian assay applicable as a large-scale screening tool for estrogenic compounds.