Summary1. Three sets of isozymes of glutathione‐S‐transferase (GST) have been identified in human tissues. They differ in their tissue distribution, incidence of genetic variation, susceptibility to inactivation byN‐ethylmaleimide and in their electrophoretic mobilities.2. The GST1 isozymes exhibit four phenotypes, including a common ‘null’ phenotype attributable to different combinations of three autosomal allelesGST1*1, GST1 *2andGST1*0of frequency 0·13, 0·23 and 0·64, respectively, in the European population.The genetic polymorphism ofGST1is easily demonstrable in adult liver, kidney, adrenal and stomach but the isozymes are only weakly expressed in skeletal and cardiac muscle and not at all in fetal liver, fibroblasts, erythrocytes, lymphocytes and platelets.3. The GST2 isozymes also exhibit variant patterns but these are probably due to post‐synthetic modification rather than allelic variation. The GST2 isozymes are not detectable in erythrocytes, platelets, cultured fibroblasts or lymphocytoid cells but are found in many other tissues, including fetal liver.4. GST3 isozymes were found as relatively strong components in every tissue examined except adult liver, with slight tissue to tissue variability in electropho