The PD-1:PD-L1 TR-FRET Assay is designed to measure the inhibition of PD-1 binding to PD-L1 in a homogeneous 384 reaction format. This FRET-based assay requires no time-consuming washing steps, making it especially suitable for high throughput screening applications. The assay procedure is straightforward and simple; a sample containing europium-labeled (Eu) PD-1, dye-labeled acceptor, biotin-labeled PD-L1, and an inhibitor is incubated for two hours. Then, the fluorescence intensity is measured using a fluorescence reader.