Abstract: Novel substrates which are amino acid 4-amino phenazones for detecting enzymes, particularly L-gamma-glutamyltransferase, are described. The amino acid gamma-glutamyl-4-aminophenazone reacts with the enzyme to produce 4-aminophenazone which is an amino dye intermediate. The amino dye intermediate is coupled with a second dye intermediate to form a chromogen. The 4-aminophenazone is particularly reacted with a phenolic naphtholic or aniline compound, particularly the phenolic compound 2-hydroxy-3,5-dichlorobenzenesulfonate, to form a red chromogen in the presence of an oxidizing agent, such as bilrubin oxidase.