Xenoestrogens, such as o,p'-DDT and octyl phenol (OP), have been associated with reproductive abnormalities in various wildlife species. Xenoestrogens mimic the natural estrogen 17 beta-estradiol and compete for binding to the estrogen receptor. Even though the affinity of o,p'-DDT and OP for the estrogen receptor is approximately 1000-fold lower than 17 beta-estradiol, the actions of xenoestrogens could be enhanced if their bioavailability in serum were greater than 17 beta-estradiol. To test this hypothesis, the yeast estrogen screen (YES) was created by expressing human estrogen receptor (hER) and two estrogen response elements (ERE) linked to the lacZ gene. The beta-galactosidase activity of the YES system was significantly increased after treatment with 17 beta-estradiol or the xenoestrogens diethylstilbestrol (DES), o,p'-DDT, and OP but not with vehicle, antiestrogen ICI 164,384, dexamethasone, or testosterone. To determine whether serum proteins affected the bioavailability of natural estrogens compared to xenoestrogens, albumin, sex hormone binding globulin (SHBG), or charcoal-stripped serum were added to the YES system and beta-galactosidase activity assayed. Albumin and SHBG decreased beta-galactosidase activity in the presence of estradiol to a greater extent than DES, o,p'-DDT, and OP. Human and alligator charcoal-stripped serum were also effective at selectively reducing beta-galactosidase activity in the presence of estradiol compared to xenoestrogens. Human serum was more effective than alligator serum in reducing beta-galactosidase activity in the presence of xenoestrogens, indicating that serum may serve as a biomarker for sensitivity to xenoestrogens. Selective binding of 17 beta-estradiol by proteins in serum indicates that certain xenoestrogens may exert greater estrogenicity than originally predicted. The estrogenic potency of a compound involves its binding affinity, bioavailability in serum, and persistence in the environment. Our data demonstrate the utility of the YES system for identifying and characterizing environmental estrogens.