1. Novel assays based on the use of the suicide repair enzyme O6-alkylguanine-DNA-alkyltransferase (AGT) to repair O6-alkylguanine-type adducts in DNA have been used for the analysis of extracts of human biopsy specimens of gastric mucosa, urinary bladder mucosa, colon and circulating lymphocytes. 2. Examination of these extracts revealed no detectable amounts of the precarcinogenic adduct O6-meG. 3. AGT measurements were the same among the normal and the autologous samples of all patients examined, which limits its prognostic value as a tumour marker. 4. AGT measurements from the cancer samples were much higher compared with the measurements of the other two groups which proves that AGT is just a marker of tumour burden. 5. Finally, AGT measurements from lymphocytes show that AGT from normal individuals have about the same value as that from patients suffering from cancer in urinary bladder mucosa and colon, but is much lower than that of patients with cancer in gastric mucosa.