Sensitive detection of K-ras oncogene mutation at codon 12 using nested polymerase chain reactions (PCR) with mismatched primers combined with selective digestion of nonmutated DNA fragments after the first PCR amplification was reported by Levi S, et al. (Cancer Res 51:3497-3502, 1991). We examined the usefulness of this novel technique in cell lines carrying various K-ras codon 12 mutations and clinical samples such as colon carcinoma tissue, corresponding normal colonic mucosa and pancreatic juice obtained from the patients with pancreatic carcinoma undergoing endoscopic retrograde pancreatography (ERP). In analysis of tumor cell lines carrying mutated K-ras at codon 12, the fragment length of the amplified DNA was 135 bp after digestion using restriction enzyme BstN1, whereas those of nonmutated cell lines were 106 bp. This method was highly sensitive to detect mutant DNA diluted at a ratio of 2048 fold with normal DNA samples obtained from peripheral blood lymphocytes. In analysis of clinical samples, 4 out of 10 colorectal carcinoma tissues were positive for K-ras gene mutation at codon 12, however, no mutations were detected in corresponding normal colonic mucosa. In analysis of pancreatic juice taken from the patients with pancreatic carcinoma, 1 out of 3 samples were positive for K-ras mutation at codon 12. Thus, this novel approach was thought to be useful for detection of minimum number of cancer cells from compound samples containing larger amounts of normal cells.