An analytical method allowing the detection of polyphenol oxidase activity on polyacrylamide gel electrophoresis (PAGE) is described. The method is rapid, sensitive and specific and is based on a coupling reaction between 4-tert-butyl-o-benzoquinone and the aromatic amine, 4-amino-N,N-diethylaniline sulphate. Catecholase activity of polyphenol oxidase appears as blue stained bands on a colourless background.