An immunohistochemical study of pi class glutathione S-transferase expression in normal human tissue. Glutathione S-transferases (GSTs), a family of isoenzymes that play an important role in protecting cells from cytotoxic and carcinogenic agents, can be separated by biochemical and immunologic characteristics into three distinct classes named alpha, mu, and pi. Previous studies have indicated that there is marked heterogeneity in the expression of different GST isoenzymes in different normal and malignant tissues. To better understand the regulation of the human pi class glutathione S-transferase isoenzyme (GST-pi), the tissue distribution of this protein wa studied by an immunohistochemical technique using an anti-GST-pi polyclonal antibody in normal paraffin-embedded human tissues. These studies indicate that there is a broad distribution of GST-pi in normal human tissues and establish a precise localization within the different organs studied. GST-pi was expressed predominantly in normal epithelial cells of the urinary, digestive, and respiratory tracts, suggesting a possible role for GST-pi in detoxication and elimination of toxic substances. Previous studies have indicated that GST-pi and the putative drug efflux pump P-glycoprotein are both overexpressed in multidrug-resistant human breast cancer cells and in xenobiotic resistant preneoplastic rat hyperplastic liver nodules. Results from this study indicate that there are also similarities between the normal tissue distribution GST-pi and that previously reported for mammalian P-glycoprotein, particularly in secretory epithelia. This finding suggests that these two gene products, which have been implicated in the development of resistance to cytotoxic drugs, may be coregulated in normal and malignant cells. 