ELISA Stop Solution is a ready-to-use solution intended for use as a stop solution in ELISA applications that use horseradish peroxidase and TMB substrate solution. As TMB substrate solution reacts with horseradish peroxidase, it produces a pronounced blue color. At the end of TMB incubation, a volume of ELISA Stop Solution equal to the volume of TMB substrate solution is added to the wells to terminate the horseradish peroxidase reaction. Addition of Stop Solution results in the rapid formation of a yellow end product. The optical density of this yellow end product is measured at 450 nm. The yellow end product is stable for sixty minutes after addition of Stop Solution and so optical density should be measured within that time.