10-acetyl-3,7-dihydroxyphenoxazin (Amplex Red) is a non-fluorescent, highly sensitive and stable probe for H2O2. Amplex Red is an applicable as a fluorogenic substrate for peroxidase. In the presence of horseradish peroxidase (HRP), the Amplex Red reagent reacts in a 1:1 stechiometry with H2O2 to produce highly fluorescent resorufin. Because H2O2 is produced in many different enzymatic reactions, the Amplex Red reagent allows researchers to detect the activity of many different enzymes. 10-acetyl-3,7-dihydroxyphenoxazin has absorption and fluorescence emission maxima of approximately 563 nm and 587 nm, respectively, and because its extinction coefficient is high (54,000 cm-1M-1), the assay can be performed either fluorometrically or spectrophotometrically.