Glutathione transferase (GST) is an enzyme capable of protecting the body from a lot of toxic compounds. Previous studies demonstrated that the erythrocyte GST (e-GST) expression increases as the level of circulating toxins increases. Aim of the present study is to verify if e-GST may represent a biomarker able to signalize an environmental pollution hazardous for humans. The study involved about 500 healthy volunteers living in eight distinct areas at or near the Sacco river valley, a region of the Frosinone district (Lazio-Italy) well known for its environmental pollution. Subjects of six areas displayed increased levels of e-GST ranging from 18% to 44% compared to 400 volunteers living in the Rome hinterland. Higher levels of GSTs are present in the areas where the risk of pollution is higher (areas 7 and 8). Interestingly, women living in the Sacco valley display much higher expression of e-GST than men, possibly due to a greater time exposition to the environmental contamination. Possible oxidative alteration of GST activity has not been observed. In conclusion, e-GST may represent an early and sensitive bio-signal of dangerous pollution for humans. (C) 2012 Elsevier Inc. All rights reserved.