Myeloperoxidase (MPO), a member of the XPO subfamily of peroxidases, is a 150 kDa heme protein produced mostly from polymorphonuclear neutrophils and monocytes. MPO is a part of the host defense system of polymorphonuclear leukocytes. It is responsible for microbicidal activity against a wide range of organisms. In the stimulated PMN, MPO catalyzes the production of hypohalous acids, primarily hypochlorous acid in physiologic situations, and other toxic intermediates that greatly enhance PMN microbicidal activity. Cathepsin S (CTSS) is one of targets of MPO, thus a functional binding ELISA assay was conducted to detect the interaction of recombinant human MPO and recombinant bovine CTSS. Briefly, biotin-linked MPO were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100 ul were then transferred to CTSS-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST 3 times and incubation with Streptavidin-HRP for 30min, then wells were aspirated and washed 5 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37℃. Finally, add 50 µl stop solution to the wells and read at 450 nm immediately. The binding activity of recombinant human MPO and recombinant bovine CTSS was shown in Figure 1, the EC50 for this effect is 0.16 ug/mL.