Kit contains the reagents necessary to perform a competition assay for identification of progesterone receptor binding compounds. An N-Terminal fusion of glutathione transferase to the ligand-binding domain of the human progesterone receptor (PR-LBD(GST)) is added to a fluorescently-tagged progesterone ligand (Fluormone™PL Green) in the presence of competitor test compounds in microwell plates. The presence of effective competitors prevents the formation of a PL Green/PR-LBD(GST) complex resulting in a decrease of the polarization value. The shift in polarization value in the presence of test compounds is used to determine relative affinity of test compounds for PR-LBD(GST).