The glutathione S-transferases are a group of enzymes that have been shown to be active in the detoxification of certain drugs, carcinogens and metabolites. Increased expression of the placental form of the enzyme has been found in association with carcinogenesis and drug resistance. Some investigators have found increased expression of the placental form of the enzyme, measured by RNA slot blot and protein assays, to be inversely related to estrogen receptor positivity in malignant breast tumors. A study is currently underway to further evaluate this relationship using immunohistochemistry to determine GST-pi expression and estrogen receptor positivity in formalin-fixed paraffin embedded breast tumors. We also plan to prospectively study fine needle aspirates of breast cancer patients to assess P-glycoprotein expression before and after chemotherapy to determine if a relationship exists between P-glycoprotein expression and response to therapy.