para-Nitrophenylphosphate (pNPP) is a non-proteinaceous chromogenic substrate for alkaline and acid phosphatases used in ELISA and conventional spectrophotometric assays. Phosphatases catalyze the hydrolysis of pNPP liberating inorganic phosphate and the conjugate base of para-nitrophenol (pNP). The resulting phenolate is yellow, with a maximal absorption at 405 nm. This property can be used to determine the activity of various phosphatases including alkaline phosphatase (AP) and protein tyrosine phosphatase (PTP).