The PolarScreen™Androgen Receptor (AR) Competitor Assay, Red uses a rat AR ligand-binding domain (AR-LBD) tagged with histidine (His) and glutathione-S-transferase (GST) and a novel, tight-binding, selective fluorescent AR ligand (Fluormone™AL Red) in a homogenous, mix-and-red assay format. AR-LBD/Fluormone™AL Red complex produces high polarization. This complex is then added to individual test compounds in microwell plates. Competitors displace the fluorescent Fluormone™AL Red Ligand from the AR-LBD/ Fluormone™AL Red complex, causing the fluorescent ligand to tumble rapidly during its fluorescence lifetime, resulting in a low polarization value. Noncompetitors will not displace the fluorescent ligand from the complex; therefore, the polarization value remains high. The shift in polarization value in the presence of the test compounds is used to determine relative affinity of test compounds for the AR-LBD.