||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 Colorimetric Screening Assay Kit is designed for screening inhibitors of PD-1 receptor binding to PD-L1 ligand. This kit comes in a convenient 96-well format, with biotin-labeled PD-1, purified PD-L1, streptavidin-labeled HRP, and assay buffer for 100 binding reactions. The key to this kit is the high affinity of biotin-labeled PD-1 for streptavidin-HRP. Only a few simple steps on a microtiter plate are required for the assay. First, PD-L1 is coated on a 96-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 a colorimetric HRP substrate to produce color, which can then be measured using a UV/Vis spectrophotometer microplate reader.