Human alpha, pi, and mu class glutathione S-transferases (GSH S-T) have been localized immunohistologically in a variety of organs. Alpha GSH S-T are found principally in hepatocytes, proximal convoluted tubules of kidney, the deep reticular layer of the adrenal gland, interstitial cells of the testis, and oxyntic cells of the stomach. The pi GSH S-T are present in relative abundance in ductular, as opposed to parenchymal cells in the liver, pancreas, salivary glands, and kidney. The presence of mu GSH S-T in the tissues of certain patients and its absence in the same tissues from other patients has been demonstrated. The pi GSH S-T seems to be most persistently and strongly expressed in tumors but alpha GSH S-T are also found in some neoplasms whereas the mu GSH S-T are occasionally present when the other two transferases are weak or absent.