BCIP, toluidine (5-Bromo-4-chloro-3-indoxyl phosphate, p-toluidine salt) is a widely used chromogenic phosphatase substrate, which forms a dark blue (λmax 615 nm) precipitate on enzymatic hydrolysis. It is often used with the oxidant NBT (nitro blue tetrazolium chloride), which facilitates the precipitation. This kit contains 100 mg each of BCIP sodium salt and the oxidant NBT (Nitro blue tetrazolium).