Plant-derived phytoestrogens and estrogens in hormone replacement therapies have overlapping yet sometimes divergent effects on the incidence of breast cancer and osteoporosis. Using human MCF-7 breast carcinoma and G-292 osteosarcoma cell lines, it was investigated whether the phytoestrogens genistein and daidzein affect reporter gene transcription via the estrogen receptors (ERs) ERα and ERβ1 as well as whether they affect the expression of estrogen-responsive genes in MCF-7 cells and the secretion of the cytokine IL-6 from G-292 cells. The results showed that genistein and daidzein potently trigger transactivation with ERβ1 from estrogen response element-reporter genes (EC50s of 1.7–16 nM) although they were 400- to 600-fold less potent than 17β-estradiol (E2) (EC50 of 0.02–0.04 nM). E2 was the only potent activator of ERα (EC50 of 0.1–0.4 nM). The rank order potency (E2 > genistein > daidzein) is maintained in MCF-7 cells as well as G-292 cells with both receptor subtypes, with a strong receptor selectivity of the phytoestrogens for ERβ1 over ERα. Genistein and daidzein increased the expression of estrogen-responsive genes in MCF-7 cells. Daidzein, like E2, inhibited IL-1β- and hormone-mediated IL-6 secretion from G-292 cells. The results provide a basis for understanding how dietary phytoestrogens protect bone without increasing the risks for breast cancer.