GSTs are ubiquitous multifunctional enzymes, which play a key role in cellular detoxification. The enzymes protect cells against toxicants by conjugating them to glutathione thereby neutralizing their electrophilic sites and rendering the products more water-soluble. Cayman’s GST Assay measures total GST activity (cytosolic and microsomal) by measuring the conjugation of 1-chloro-2,4-dinitrobenzene (CDNB) with reduced glutathione. The conjugation is accompanied by an increase in absorbance at 340 nm. Cayman’s GST Assay can be used to measure GST activity in plasma, erythrocyte lysates, tissue homogenates, and cell lysates. Cytosolic and microsomal GST activity can also be individually assayed.