Immunohistochemical studies of the developmental expression of the Alpha- and Pi-class glutathione S-transferases in human liver have shown that the Pi enzyme is expressed in bile-duct epithelium and some hepatocytes but not in haematopoietic cells. This locus is down-regulated during gestation in hepatocytes but not in epithelium. The enzymes of the Alpha set were also found in only some hepatocytes, and it appears that many cells express neither these nor the Pi forms.