Cathepsin S (CTSS, EC 3.4.22.27) is a lysosomal cysteine proteinase that is suggested to participate in the degradation of antigenic proteins to peptides for presentation on MHC class II molecules. Biovision’s Cathepsin S Inhibitor Screening Kit utilizes the ability of Cathepsin S to cleave the synthetic AFC based peptide substrate to release AFC, which can be easily quantified using a fluorometer or fluorescence microplate reader. In the presence of a Cathepsin S inhibitor, the cleavage of the substrate is reduced/abolished resulting in decrease or total loss of the AFC fluorescence. This high-throughput adaptable assay kit is simple, sensitive, and rapid tool to screen the potential inhibitors of Cathepsin S.