Abstract: The present invention relates to a method of using a storage-stable, pre-stained 3,3′,5,5′-tetramethylbenzidine (TMB) substrate in an enzyme substrate. The TMB substrates comprise: (a) a water-based TMB substrate composition which includes a peroxide, e.g., hydrogen peroxide or urea hydrogen peroxide, and (b) a dye which is soluble therein. Suitable dyes to be included in the substrate preferably have an absorbance at 450 nm of at the most milli absorbance units. Examples of dues are Pholixine B, Eosin B and Quinaldine Red. The substrates are advantageous in that they include a dye which is visible to the human eye. This is especially valuable when the substrates are handled and measured. The substrates are especially suitable for enzyme assays such as enzyme-linked-immunosorbent-assays (ELISA), e.g. horse radish peroxide (HRP) is used. The substrates may advantageously include less than 5% organic solvents and a solubility increasing agent such as polyvinylalcohol.