Abstract: This invention relates to enzyme assays and substrates for use in such assays. The presence or absence of certain enzymes in vivo is a useful indicator of illness or deficiency in the organisms concerned. Enzymes are also useful in monitoring microbial growth in fermentors and in the food industry and are important in enzyme-linked immunosorbent assays (ELISA), and in the characterization of bacterial species in culture. Disclosed is a reagent for enzyme assay comprising a substrate consonant with a given enzyme to be assayed, labelled with a chromogenic group which, on action of the given enzyme, is liberated to give a colored product. Also disclosed is a kit for the assay of an enzyme comprising the reagent of the disclosed invention.