The glutathione S-transferase gene (GST pi) is located on the same chromosome band (11q13) as proto-oncogenes INT2 and HSTF1 which are frequently amplified in breast cancer. Using the Southern blot technique, we looked for the amplification of the GST pi gene in 17 fresh tumors from human mammary carcinoma. The tumors were preselected because either they had an amplification of the INT2 proto-oncogene detected by dot blot, or their karyotypes exhibited or did not exhibit homogeneously staining regions, a cytogenetic character indicating amplification. Coamplification of GST pi, HSTF1 and INT2 was observed in five tumors, and coamplification of GST pi and HSTF1 without amplification of INT2 in another tumor. We also observed coamplification of GST pi, INT2, HSTF1 in the mammary carcinoma cell line MDA/MB134, whereas GST pi alone was amplified in the mammary epithelial cell line HBL100. These results indicate that INT2, HSTF1 and GST pi belong to the same large amplicon. Since GST pi is involved in intracellular detoxication and since chemotherapeutic drugs are among its substrates, it will be of interest to study GST pi gene expression as well as the response to chemotherapy in patients presenting this amplicon.