Cayman’s Acid Phosphatase Assay provides a convenient method for detecting AP activity in plasma, serum, urine, and semen. The assay utilizes para-nitrophenyl phosphate (pNPP) as a chromogenic substrate for the enzyme. In the first step, AP dephosphorylates pNPP. In the second step, the phenolic OH-group is deprotonated under alkaline conditions resulting in p-nitrophenolate that yields an intense yellow color which can be measured at 405-414 nm. The kit provides all reagents needed to assay AP activity, and includes L-tartrate, an inhibitor of non-tartrate resistant APs.