Zearalenone, a secondary metabolite produced by several plant-pathogenic fungi of the genus Fusarium, has high estrogenic activity in vertebrates. We developed a Saccharomyces cerevisiae bioassay strain that we used to identify plant genes encoding UDP-glucosyltransferases that can convert zearalenone into zearalenone-4-O-glucoside (ZON-4-O-Glc). Attachment of the glucose moiety to zearalenone prevented the interaction of the mycotoxin with the human estrogen receptor. We found that two of six clustered, similar UGT73C genes of Arabidopsis thaliana encode glucosyltransferases that can inactivate zearalenone in the yeast bioassay. The formation of glucose conjugates seems to be an important plant mechanism for coping with zearalenone but may result in significant amounts of "masked" zearalenone in Fusarium-infected plant products. Due to the unavailability of an analytical standard, the ZON-4-O-Glc is not measured in routine analytical procedures, even though it can be converted back to active zearalenone in the digestive tracts of animals. Zearalenone added to yeast transformed with UGT73C6 was converted rapidly and efficiently to ZON-4-O-Glc, suggesting that the cloned UDP-glucosyltransferase could be used to produce reference glucosides of zearalenone and its derivatives.