Glutathione S-transferases (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. The glutathione conjugates are metabolized further to mercapturic acid and then excreted. Based on their sequence homology, substrate specificity, and immunological cross-reactivity, GSTs have been grouped into species-independent classes of isozymes. These classes are comprised of cytosolic enzymes and the fifth form is microsomal. The Cayman Chemical Glutathione S-Transferase Assay Kit 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. The rate of increase is directly proportional to the GST activity in the sample. The Cayman GST Assay Kit can be used to measure GST activity in plasma, erythrocyte lysates, tissue homogenates, and cell lysates. Cytosolic and microsomal GST activity can also be assayed separately.