A number of chemical pollutants have physiological effects mimicking those of estrogen. These xenobiotic estrogens pose an insidious risk to wildlife and humans by disrupting reproductive and developmental processes, thereby impairing both the exposed individuals and their offspring. Xenobiotic estrogens are impacting both wildlife and human health, thus it is important to screen chemicals for estrogenic potential, and to monitor environmental levels of estrogenic pollutants. Although most known xenobiotic estrogens show little structural similarity, they do produce predictable physiological responses. This allows the use of functional estrogenicity assays employing specific biomarkers of estrogen action, such as vitellogenin. Vitellogenin is an egg-yolk precursor protein produced by the liver in response to estrogens and estrogen agonists. Vitellogenin is normally found only in the serum of adult female oviparous vertebrates, but it can be induced in males and immature females by estrogen. Vitellogenin induction bioassays can be used to screen chemicals for estrogenic and antiestrogenic activity, to test water for the presence of xenobiotic estrogens, and to screen wildlife populations for exposure to environmental estrogens.