Used primarily as a chromogenic substrate for the histochemical and cytochemical detection of non-specific esterase activity. It is hydrolyzed by esterases to release 5-bromo-4-chloroindoxyl, which spontaneously dimerizes and oxidizes to form an insoluble, intensely blue-green indigo precipitate at the site of enzyme activity. This makes it valuable in pathology, hematology, immunology, and microbiology for visualizing esterase distribution in tissues, cells, and bacteria. Applications include diagnostic assays for identifying esterase-positive leukocytes and microorganisms.