Cell signaling through the PD-1 receptor upon binding the PD-L2 ligand attenuates immune responses and is exploited by both tumors and viruses. The PD- 1[Biotinylated]:PD-L2 Inhibitor Screening Colorimetric Assay Kit is designed for screening inhibitors of PD-1 receptor binding to PD-L2 ligand. 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-L2 is coated on a 96-well plate. Next, PD-1 is incubated with PD-L2 on the plate. Finally, the plate is treated with streptavidin-HRP followed by addition of a colorimetric HRP substrate substrate to produce color, which can then be measured using a UV/Vis spectrophotometer microplate reader.