Glutathione S-transferase pi, a drug-detoxifying isoenzyme of potential prognostic value for subsets of patients with mammary cancer, was studied by immunohistochemistry and Southern blot analysis in 58 infiltrating ductal carcinomas. The results were compared with the findings of six important clinicopathologic parameters. Cytoplasmic immunoreactivity for glutathione S-transferase pi was absent in 40%, 1+ in 26%, 2+ in 15%, and 3+ in 19% of the cases. There were no significant correlations between the level of immunostaining and patient age, tumor size, axillary lymph node status, nuclear pleomorphism, or tubule formation, but there was a trend with mitotic count (P = 0.06). Immunoreactivity was associated with histologic grade (P = 0.02) and inversely correlated with estrogen (P = 0.006) and progesterone (P = 0.02) receptor content. Ten percent of the cases showed modest levels of glutathione S-transferase pi gene amplification, but no significant correlations were observed between glutathione S-transferase pi amplification and any of the clinicopathologic parameters or level of immunostaining. The results indicate that increased immunohistochemical staining for glutathione S-transferase pi occurs in high-grade, estrogen and progesterone receptor-negative neoplasms. As glutathione S-transferase pi gene amplification appears unassociated with immunopositivity, other mechanisms are responsible for the production of this isoenzyme in infiltrating ductal carcinomas.