The human estrogen receptors (hER) and hER, differentially expressed and localized in various tissues and cell types, mediate transcriptional activation of target genes. These encode a variety of physiological reproductive and nonreproductive functions involved in energy metabolism, salt balance, immune system, development, and differentiation. As a step toward developing a screening assay for the use in applications where significant numbers of compounds or complex matrices need to be tested for (anti) estrogenic bioactivity, hER and hER were expressed in a genetically modified Saccharomyces cerevisiae strain, devoid of three endogenous xenobiotic transporters (PDR5, SNQ2, and YOR1). By using receptor-mediated transcriptional activation of the green fluorescent protein optimized for expression in yeast (yEGFP) as reporter 17 natural, comprising estrogens and phytoestrogens or synthetic compounds among which tibolone with its metabolites, gestagens, and antiestrogens were investigated. The reporter assay deployed a simple and robust protocol for the rapid detection of estrogenic effects within a 96-well microplate format. Results were expressed as effective concentrations (EC₅₀) and correlated to other yeast based and cell line assays. Tibolone and its metabolites exerted clear estrogenic effects, though considerably less potent than all other natural and synthetic compounds. For the blood serum of two volunteers, considerable higher total estrogenic bioactivity than single estradiol concentrations as determined by immunoassay was found. Visualization of a hER/GFP fusion protein in yeast revealed a sub cellular cytosolic localization. This study demonstrates the versatility of (anti) estrogenic bioactivity determination using sensitized S. cerevisiae cells to assess estrogenic exposure and effects.--Hasenbrink, G., Sievernich, A., Wildt, L., Ludwig, J., and Lichtenberg-Fraté, H. Estrogenic effects of natural and synthetic compunds including tibolone assessed in Saccharomyces cerevisiae expressing the human estrogen and receptors.