Glutathione-S-transferases from liver and erythrocytes have been separated by starch gel electrophoresis and localized by a specific staining procedure. The data suggest that the most active glutathione-S-transferases in liver are the products of two autosomal loci, GST1 and GST2. Both these loci are polymorphic, and there is evidence that a common null allele exists at the GST1 locus. The glutathione-S-transferase expressed in erythrocytes is the product of a third locus, GST3, and is not polymorphic.