Glutathione S-transferase (GST) represents a multifunctional enzyme family consisting of four known cytosolic isoforms (alpha, mu, pi, and Phi) that detoxify a variety of xenobiotic chemicals and may confer resistance to both chemotherapeutic drugs and carcinogens in various experimental models. GST-pi has already been extensively studied in clinical specimens, including breast cancer. We studied the immuno-histochemical distribution and relative immunopositivity of GST-alpha and GST-mu, based on a grading system for immunointensity, in samples of 51 neoplastic and 46 normal breast samples and 12 lymph node metastases from patients treated with intensive chemotherapy and bone marrow transplant. In normal breast tissue, GST-alpha localized predominantly to the cytoplasm of scattered cells lining the luminal aspects of the ducts. Occasional cells showed both cytoplasmic and nuclear GST-alpha immunoreactivity. GST-mu was stained in myoepithelial cells preferentially as well as in occasional ductal cells (including apocrine epithelium), vascular smooth muscle, and plasma cells. GST-alpha and GST-mu were detected in 22 of 51 (43%) and 24 of 48 (50%) invasive cancers, respectively. In paired samples of normal and malignant tissue from the same patient, GST-alpha immunostaining in cancers was significantly less intense compared to that of normal breast tissue in 13 of 41 (32%) cases. No such trend was found for GST-mu in paired samples. Neither GST-alpha nor GST-mu immunopositivity in tumor or nonneoplastic breast was found to correlate with relapse-free or overall survival in this clinical context; however, the apparent decreased expression of GST-alpha in malignant versus normal breast epithelial cells could have important implications in breast carcinogenesis.