A method of detecting β-D-glucuronidase activity in a liquid medium comprising the steps of: a) contacting a metal compound and a β-D-glucuronidase substrate according to the following formula I with a substance suspected of containing or producing a glucuronidase: wherein X is independently selected from the group consisting of H, CH 3, C 2 -C 6 alkyl, a metal cation and a non-metal cation; wherein Y is independently selected from the group consisting of H, a metal cation and a non-metal cation; wherein either i) R 1 is H or OH and R 4 is H or NO 2; or ii) R 1 and R 4 are independently selected from the group consisting of H, C 1 -C 8 alkyl, C 1 -C 8 alkoxy, C 1 -C 8 hydroxyalkyl, halogen, nitro, C 2 -C 24 acyl, C 2 -C 24 acyloxy, C 7 -C 24 aralkyl, C 6 -C 24 aryl, sulfonyl and amido; and wherein R 2 and R 3 are independently selected from the group consisting of H, OH, C 1 -C 8 alkyl, C 1 -C S alkoxy, C 1 -C 8 hydroxyalkyl, halogen, nitro, C 2 -C 24 acyl, C 2 -C 24 acyloxy, C 7 -C 24 aralkyl, C 6 -C 24 aryl, sulfonyl and amido provided that R 2 and R 3 do not both represent OH, or R 2 and R 3 together form the group represented by the formula VI: -CR 5 =CR 6 -CR 7 =CR 8 - VI; wherein R 5 -R 8 are independently selected from the group consisting of H, C 1 -C 8 alkyl, C 1 -C 8 alkoxy, C 1 -C 8 hydroxyalkyl, halogen, nitro, C 2 -C 24 acyl, C 2 -C 24 acyloxy, C 7 -C 24 aralkyl, C 6 -C 24 aryl, hydroxyl, sulfonyl and amido provided that in the group VI any vicinal groups R 5 to R 8 are not both OH; such that a product of substrate cleavage is capable of chelating the metal compound, thereby forming a coloured compound; and b) detecting the presence of the coloured compound. a) contacting a metal compound and a β-D-glucuronidase substrate according to the following formula I with a substance suspected of containing or producing a glucuronidase: wherein X is independently selected from the group consisting of H, CH 3, C 2 -C 6 alkyl, a metal cation and a non-metal cation; wherein Y is independently selected from the group consisting of H, a metal cation and a non-metal cation; wherein either i) R 1 is H or OH and R 4 is H or NO 2; or ii) R 1 and R 4 are independently selected from the group consisting of H, C 1 -C 8 alkyl, C 1 -C 8 alkoxy, C 1 -C 8 hydroxyalkyl, halogen, nitro, C 2 -C 24 acyl, C 2 -C 24 acyloxy, C 7 -C 24 aralkyl, C 6 -C 24 aryl, sulfonyl and amido; and wherein R 2 and R 3 are independently selected from the group consisting of H, OH, C 1 -C 8 alkyl, C 1 -C S alkoxy, C 1 -C 8 hydroxyalkyl, halogen, nitro, C 2 -C 24 acyl, C 2 -C 24 acyloxy, C 7 -C 24 aralkyl, C 6 -C 24 aryl, sulfonyl and amido provided that R 2 and R 3 do not both represent OH, or R 2 and R 3 together form the group represented by the formula VI: -CR 5 =CR 6 -CR 7 =CR 8 - VI; wherein R 5 -R 8 are independently selected from the group consisting of H, C 1 -C 8 alkyl, C 1 -C 8 alkoxy, C 1 -C 8 hydroxyalkyl, halogen, nitro, C 2 -C 24 acyl, C 2 -C 24 acyloxy, C 7 -C 24 aralkyl, C 6 -C 24 aryl, hydroxyl, sulfonyl and amido provided that in the group VI any vicinal groups R 5 to R 8 are not both OH; such that a product of substrate cleavage is capable of chelating the metal compound, thereby forming a coloured compound; and i) R 1 is H or OH and R 4 is H or NO 2; or ii) R 1 and R 4 are independently selected from the group consisting of H, C 1 -C 8 alkyl, C 1 -C 8 alkoxy, C 1 -C 8 hydroxyalkyl, halogen, nitro, C 2 -C 24 acyl, C 2 -C 24 acyloxy, C 7 -C 24 aralkyl, C 6 -C 24 aryl, sulfonyl and amido; and b) detecting the presence of the coloured compound.