The 1-Step™ ABTS (2,2'-azinobis [3-ethylbenzthiazoline-6-sulfonic acid] diammonium salt) is a ready-to-use, water-soluble ELISA substrate for horseradish peroxidase. This substrate yields a green product with absorbance maxima at 410 nm and 650 nm. Adding SDS to the substrate stops the reaction without changing the colour.