The level of serum Glutathioe S-transferase (GSTs) was first measured in 224 female patients with breast cancer and 17 patients with benign breast tumor and 96 normal female subjects. The relationship between serum GSTs and biological characteristics of breast cancer was studied. The mean serum GSTs in patients with breast cancer was 1.22 +/- 1.44 ng/ml. The positive rate was 51.8%. The level of serum GSTs in the patients with breast cancer was significantly higher than in normal subjects and patients with benign breast tumor. No correlation was found with regard to the level of serum GSTs, the size of the breast cancer, stage, lymphatic metastasis and estrogen receptor status. Serum GSTs level is thus of little value in the evaluation of response and prognosis of breast cancer.