Standardized, automated ligand-binding assays facilitate evaluation of endocrine activities of environmental chemicals and identification of antagonists of nuclear receptor ligands. Many current assays rely on fluorescently labeled ligands that are significantly different from the native ligands. The authors describe a radiolabeled ligand competition scintillation proximity assay (SPA) for the androgen receptor (AR) using Ni-coated 384-well FlashPlates and liganded AR-LBD protein. This highly reproducible, low-cost assay is well suited for automated high-throughput screening. In addition, the authors show that this assay can be adapted to measure ligand affinities for other nuclear receptors (peroxisome proliferation-activated receptor gamma, thyroid receptors alpha and beta).