In order to identify phytoprogestins from T. pratense , we obtained a standardized phase IIred clover extract. This extract was previously characterized by the Botanical Center at theUniversity of Illinois at Chicago, and they described the identification of 23 components ofisoflavone, flavonoid, pterocarpin, and coumarin structural classes. 9 In the present study,Ishikawa cells that stably express progesterone receptor B (PR-B) were used to screen forcompounds that activate or inhibit the receptor. Inhibition using this assay is based on thecompound’s ability to block activation of the progesterone receptor in the presence ofprogesterone. Initially, 60 μg/mL of the red clover extract decreased luciferase activity by44% ( p 90%.Surprisingly, 50 μM of irilone (1) resulted in an increase in PRE/Luc activity over thatinduced by 100 nM progesterone alone, increasing PRE/Luc induction from 25.7-fold to48.4-fold ( p