The adenomatous polyposis coli (APC) gene, whose mutations are responsible for familial adenomatous polyposis, is a major negative controller of the Wnt/beta-catenin pathway. To investigate the dose-dependent effects of APC protein in suppressing intestinal tumorigenesis, we constructed mutant mice carrying hypomorphic Apc alleles Apc(neoR) and Apc(neoF) whose expression levels were reduced to 20% and 10% of the wild type, respectively. Although both hypomorphic heterozygotes developed intestinal polyps, tumor multiplicities were much lower than that in Apc(Delta716) mice, heterozygotes of an Apc null allele. Like in Apc(Delta716) mice, loss of the wild-type Apc allele was confirmed for all polyps examined in the Apc(neoR) and Apc(neoF) mice. In the embryonic stem cells homozygous for these hypomorphic Apc alleles, the level of the APC protein was inversely correlated with both the beta-catenin accumulation and beta-catenin/T-cell factor transcriptional activity. These results suggest that the reduced APC protein level increases intestinal polyp multiplicity through quantitative stimulation of the beta-catenin/T-cell factor transcription. We further estimated the threshold of APC protein level that forms one polyp per mouse as approximately 15% of the wild type. These results also suggest therapeutic implications concerning Wnt signaling inhibitors.