Resazurin (also known as alamarBlue™) is the N-oxide of the fluorescent dye resorufin and is useful for detecting reductive activities in cells. It has been widely used for measuring cell proliferation (1,2) and mitochondrial metabolic activity. Resazurin itself is non-fluorescent until it is reduced to the highly red fluorescent resorufin (λEx/λEm: 563/587 nm).