It was previously hypothesized that serum gamma-glutamyltransferase (GGT) within its reference range predicts various clinical outcomes as a sensitive marker of oxidative stress in humans. This study further hypothesizes that serum GGT can mark exposure to various environmental pollutants, based both on recent epidemiological findings and on well-established biochemical features of cellular GGT. Cellular GGT is a prerequisite for metabolism of GSH conjugates that detoxify xenobiotics to mercapturic acid. Under this concept, serum GGT may increase with increasing exposure to environmental pollutants which need to be conjugated to GSH. Supporting this concept, it was recently reported that serum GGT within its reference range was linearly associated with important environmental pollutants, including lead, cadmium, dioxin and organochlorine pesticides. As a marker of the amount of conjugated xenobiotics, recent epidemiological findings about serum GGT imply the possibility of harmful effects of various environmental pollutants at background levels currently regarded as safe.