Naphthol AS-BI sulphate potassium salt is a diazo dye that is used as a substrate for enzymes. It is usually used in conjunction with other substrates, such as o-phenylenediamine and sodium nitroprusside. The naphthol AS-BI sulphate potassium salt has been shown to be an effective stain in the detection of malignant cells. Naphthol AS-BI sulphate potassium salt has also been shown to be a useful tool in environmental testing and food testing due to its high purity and high quality.