Phytoestrogens exert pleiotropic effects on cellular signaling and show some beneficial effects on estrogen-dependent diseases. However, due to activation/inhibition of the estrogen receptors ERa or ERb, these compounds may induce or inhibit estrogen action and, therefore, have the potential to disrupt estrogen signaling. We performed a comprehensive analysis and potency comparison of phytoestrogens and their humanmetabolites for ER binding, induction/ suppression of ERa and ERb transactivation, and coactivator recruitment in human cells. The soy-derived genistein, coumestrol, and equol displayed a preference for transactivation of ERb compared to ERa and were 10to 100-fold less potent than diethylstilbestrol. In contrast, zearalenone was themost potent phytoestrogen tested and activated preferentially ERa. All other phytoestrogens tested, including resveratrol andhumanmetabolites of daidzein and enterolactone, were weak ER agonists. Interestingly, the daidzein metabolites 30,40,7-isoflavone and 40,6,7-isoflavone were superagonists on ERa and ERb. All phytoestrogens tested showed reduced potencies to activate ERa and ERb compared to diethylstilbestrol on the estrogen-responsive C3 promoter compared to a consensus estrogen response element indicating a degree of promoter dependency. Zearalenone and resveratrol were antagonistic on both ERa andERb at high doses. The phytoestrogens enhanced preferentially recruitment of GRIP1 to ERa similar to 17b-estradiol. In contrast, for ERb no distinct preference for one coactivator (GRIP1 or SRC1) was apparent and the overall coactivator association was less pronounced than for ERa. Due to their abundance and (anti)-estrogenicpotencies, the soy-derived isoflavones, coumestrol, resveratrol, and zearalenone would appear to have the potential for effectively functioning as endocrine disruptors.