The classical mouse uterine bioassay was evaluated and adapted for routine diagnostic use in response to requests for evaluation of forages suspected of being estrogenic. Forages were extracted in acetone or 10% ethanol in acetone (v/v). Extracts were mixed with ground corn-based mouse feed. Immature female mice (n = 3/group) were fed a total of 100 g of the ground feed for 5 days. Body weights were monitored before and after the trial. After 6 days, the mice were euthanized and uterine weights were determined. Mean uterine weights were compared using 1-way analysis of variance with preselected contrasts for individual means. Selected uteruses were fixed in 10% neutral buffered formalin for histologic examination. Control feeds, diethylstilbestrol (DES), estradiol, coumestrol, feeds with no reported estrogenic properties, and a feed that caused hyperestrogenism in cattle were tested. Moderate levels of estrogenic compounds resulted in dose-responsive uterine enlargements (10-270 ppm coumestrol over 5 days). Extremely high levels of estrogen frequently resulted in feed refusal and lack of uterine enlargement (10 ppm DES, 100 ppm estradiol). Diagnostically significant estrogenic activity was recovered from the feed known to have been estrogenic in cattle. The classical mouse uterine bioassay was relatively inexpensive, quick, repeatable, and capable of detecting clinically relevant coumestrol levels in hay.