In order to clarify the expression of cytochrome P450 and glutathione S-transferase in human esophagus, 41 samples of human esophagus with squamous-cell carcinoma were investigated by immunoblot analysis and enzyme assays. Cytochrome P450 1A2/1 was clearly expressed in microsomes, and the amount in samples with tumorous tissue was significantly greater than that in samples without tumourous tissues or in liver; cytochrome P450 2B6 and 3A4/3 were expressed polymorphically. Aryl hydrocarbon hydroxylase activity was detected in microsomes and was greater in samples from smokers than non-smokers. Patients who both smoked and drank alcohol, however, had activity similar to that of patients without these habits. Glutathione S-transferase M1 and A1/2 protein existed polymorphically in cytosol, and glutathione S-transferase P1-1 was detected in all samples. The frequency of expression of the glutathione S-transferase A1/2 protein was greater in patients with M1 protein than in those without; no difference in the expression was seen for glutathione S-transferase P1-1. Neither smoking nor drinking influenced the expression or activity of glutathione S-transferase. Our data support the idea that some carcinogens can be directly activated or inactivated in human esophageal epithelium.