2,2′-Azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) is a peroxidase substrate suitable for use in ELISA (enzyme-linked immunosorbent assay) procedures. This substrate produces a soluble end product that is green in color and can be read spectrophotometrically at 405nm. The reaction may be stopped with 1% sodium dodecyl sulfate (SDS). It is recommended for ELISA (microwell) procedures, not recommended for membrane applications.