p-Nitroblue Tetrazolium Chloride is an NADPH-diaphorase substrate that competitively inhibits NOS (nitric oxide synthase) (IC50 = 3-4 μM). p-Nitroblue Tetrazolium Chloride is a well-known scavenger of superoxide anions and is useful as a substrate for alkaline phosphatase, often in conjunction with BCIP.