We have demonstrated previously that intestinal tumor formation in B6 Min/+ mice is always accompanied by loss of the wild-type adenomatous polyoposis coli (Apc) allele and that intestinal tumor multiplicity in B6 Min/+ mice can be significantly increased by treatment with a single dose of N-ethyl-N-nitrosourea (ENU). Here, we show that some tumors from ENU-treated Min/+ mice can form without complete elimination of Apc⁺. At least 25% of these tumors acquired somatic Apc truncation mutations. Interestingly, some ENU-induced tumors demonstrated loss of the Apc⁺ allelic marker examined by the quantitative PCR assay used here. Using two methods of mutation detection, we identified no Apc mutations in at least 12% of the tumors from ENU-treated B6 Min/+ mice. Finally, no H- or K-ras-activating mutations were detected in intestinal tumors from either untreated or ENU-treated Min/+ mice. The majority of somatic human APC mutations in intestinal tumors lead to APC truncation. Our results demonstrate that somatic Apc truncation mutations also frequently occur in ENU-induced intestinal tumors in Min mice.