Glutathione S-transferases in breast tissue play an important role in the susceptibility to the mutagenic effects of chemical carcinogens and in the response of breast tumors to chemotherapy. In this study the immunohistochemical staining characteristics of glutathione S-transferase isoenzymes (alpha, mu, pi, and theta) were investigated in invasive duct carcinomas and in normal breast tissue of 43 patients. The relationships between the expression of the GST isoenzymes and some clinicopathological features were also examined. Diffuse cytoplasmic staining of varying intensity was observed for GST alpha, theta, and pi in normal and tumorous breast tissue in 100% of the samples. In normal epithelium there was a stronger intensity of staining for GST alpha, mu. and pi expression than in invasive tumor tissues (P < 0.05): however, it was statistically proven that in normal and tumor epithelial cells there was no significant difference in the GST theta isoenzyme staining scores (P > 0.05). In this study significant relationships were observed between microcalcification status and GST mu, between menopause status and GST alpha, and between tumor grade and GST mu expression (P < 0.05). The relationships between GST isoenzyme expression. and estrogen receptor status, tumor grade, smoking status, chemotherapy status. parity. patient's age, and hormone therapy status were not statistically significant (P > 0.05).