Germ-line mutations in the adenomatous polyposis coli (APC) gene are responsible for familial adenomatous polyposis (FAP). In the present study, we have used the protein truncation test to screen for mutations in exon 15 and exons 1-14 of the APC gene and denaturing gradient gel electrophoresis to analyze exons 1-14. We have studied nine unrelated FAP kindreds, eight with the classical phenotype and one with an atypical phenotype, with several family members exhibiting fewer than 50 colonic polyps. The combined use of these two methodologies allowed the identification of seven novel mutations, with two unrelated families sharing the same mutation. All mutations were chain terminating: six resulted from small deletions, one from a small insertion, and one was a point mutation, resulting in a premature stop codon. Seven mutations were located in exon 15 of the APC gene, one was in exon 10, and the remaining one, which corresponded to the kindred with an atypical phenotype, was located in exon 4.