Cell signaling through the PD-1 receptor upon binding the PD-L1 ligａnd attenuates immune responses and is exploited by both tumors and viruses. The PD-1[Biotinylated]: PD-L1 Inhibitor Screening Assay Kit is designed for screening and profiling inhibitors of this signaling. This kit comes in a convenient 384-well format with biotin-labeled PD-1, purified PD-L1, streptavidin-labeled HRP, and assay buffer for 400 binding reactions. The key to this kit is the high sensitivity of detection of biotin-labeled PD-1 by streptavidin-HRP. Only a few simple steps on a microtiter plate are required for the assay. First, PD-L1 is coated on a 384-well plate. Next, PD-1 is incubated with PD-L1 on the plate. Finally, the plate is treated with streptavidin-HRP followed by addition of an HRP substrate to produce chemiluminescence, which can then be measured using a chemiluminescence reader.