L-Arginyl-L-arginine 7-amido-4-methylcoumarin trihydrochloride (LAA) is a chemiluminescent substrate that can be used in diagnostic assays for the detection of enzymes and other biomolecules. LAA is a chromogenic substrate which generates light upon addition of hydrogen peroxide and an oxidase, such as horseradish peroxidase or myeloperoxidase. The emission spectrum of LAA ranges from green to blue, depending on the pH. This compound has been shown to be stable up to pH 10 and up to 60 degrees Celsius.