Duodenal adenomas occur almost inevitably in patients with familial adenomatous polyposis (FAP) whereas gastric adenomas are rare. FAP patients are also at high risk of duodenal cancer. Within the duodenum, adenomas cluster around the ampulla of Vater, as do the majority of duodenal cancers, suggesting that bile plays a role in tumour development. We therefore tested duodenal bile from 29 postcolectomy FAP patients (27 of whom had duodenal adenomas) and 24 non-FAP patients for mutagenicity, using techniques that detect point mutations in bacteria. Results which appeared to show that FAP bile was more mutagenic than control bile could be accounted for by a feeding effect, elimination of which also eliminated 'mutagenicity'. Under the conditions of our assays we conclude that if bile is an important factor in genesis of duodenal tumours, it does not act by inducing point mutation.