Evaluating endocrine activities of environmental chemicals or screening for new small molecule modulators of the androgen receptor (AR) transcription activity requires standardized and reliable assay procedures. Scintillation proximity assays (SPA) are sensitive and reliable techniques that are suitable for ligand competition binding assays. We have utilized a radiolabeled ligand competition binding assay for the androgen receptor (AR) that can be carried out in a 384-well format. This standardized, highly reproducible and low-cost assay has been automated for high-throughput screening (HTS) purposes.