Altered expression of the glutathione S-transferases (GSTs) has been implicated in the progression to tumour after exposure to carcinogens, and GST Pi has been suggested as a possible marker of preneoplasia in the cervix. We have studied expression of the GST isoenzymes in normal cervix, non-dysplastic cervical condylomata, cervical intraepithelial neoplasia (CIN), and invasive squamous carcinoma of the cervix using immunocytochemistry. An increase in GST Pi in CIN as compared with normal cervix was paralleled by a reduction in the expression of microsomal GST. Similar changes were seen in cervical condylomata and immature squamous metaplasia, and thus neither isoenzyme is a marker of dysplasia. Microsomal GST was expressed in only 66 per cent of cases and in 22 per cent showed strong expression in vascular endothelium. These findings are of particular interest in view of the association between cervical carcinoma and cigarette smoking. Differences between individuals in the ability to detoxify environmental carcinogens may influence the likelihood of progression from benign proliferation to invasive malignancy.