The expression of glutathione S-transferase mu was measured by a qualitative immunoenzymatic assay in the blood samples of 108 women (63 breast cancer patients and 45 healthy controls) in order to analyze the relationships of GST-mu phenotype and smoking habits with tumor characteristics of breast cancer patients, such as tumor extension, nodal status, hormone receptor status and DNA content by flow cytometry. GST-mu was expressed in 53/108 (49%) of cases without any significant difference between healthy or neoplastic subjects, smokers or non-smokers, pre or post-menopausal, younger or older subjects. Moreover, the percentages of the GST-mu phenotype did not differ significantly in patients with different ER and PgR tumor status, tumor extension or nodal status. By contrast, aneuploid DNA tumor content was shown to be significantly associated with GST-mu expression (24% and 76% GST-mu positive, respectively, in diploid and aneuploid cases; p < 0.003). The biological meaning of this association remains to be interpreted.