Cathepsin G (CatG) is an endopeptidase that is associated with the early immune response. The synthetic compound cathepsin G inhibitor I (CGI-I) was tested for its ability to inhibit the activity of CatG via a new surface plasmon resonance imaging assay. CGI-I was immobilized on the gold surface of an SPR sensor that was first modified with 1-octadecanethiol. A concentration of CGI-I equal to 4.0 μg·mL-1 and a pH of 8.0 were found to give the best results. The dynamic response of the sensor ranges from 0.25 to 1.5 ng·mL-1, and the detection limit is 0.12 ng·mL-1. The sensor was applied to detect CatG in human saliva and white blood cells.FigureThe synthetic compound cathepsin G inhibitor I (CGI-I) was tested for its ability to inhibit the activity of cathepsin G via a newly developed surface plasmon resonance imaging assay. The sensor was applied to detect cathepsin G in human saliva and white blood cells.