STAT (signal transducers and activators of transcription) is a class of transcription factors that are activated upon cytokine or growth factor binding to cell surface receptors. Activated STAT proteins dimerize, translocate into the nucleus, and activate transcription, leading to various immune responses. The inhibition of STAT binding to cell receptors might provide a means to modulate these immune responses. We developed a high-throughput biochemical assay to measure the interaction between an interferon-gamma receptor-derived phosphotyrosine-containing peptide and STAT1, using fluorescence polarization as the detection method. This assay can be used to screen for small molecules capable of disrupting receptor-STAT interactions.