5-Cyano-2,3-di-(p-tolyl)tetrazolium (chloride) is a redox-sensitive tetrazolium salt used primarily to detect metabolic activity in microorganisms. This membrane-permeable indicator is readily taken up by living cells and reduced by respiratory activity to produce a red fluorescent, water-insoluble formazan crystal (absorption maximum at 450 nm). While the oxidized salt is non-fluorescent, the reduced formazan can be evaluated using a standard rhodamine filter set (excitation: 510-560 nm; emission > 590 nm), although optimum excitation requires a blue 420 nm excitation filter. For flow cytometry, a 480 nm excitation laser combined with detection in the red region is suitable.