Min mice are heterozygous for a nonsense mutation in the murine adenomatous polyposis coli (APC) gene and spontaneously develop multiple intestinal neoplasms similar to the familial adenomatous polyposis (FAP) syndrome in humans. Aberrant crypt foci (ACF) are assumed to be preneoplastic lesions in both murine and human colon carcinogenesis and have been observed in FAP patients. Light microscopic examination of the colonic mucosa of 42 Min mice did not show even a single 'classical' ACF on the basis of previously defined criteria, specifying that they are elevated above the surrounding mucosa. However, in Min mice we discovered aberrant crypt foci of a different type, which we denoted ACF(Min). In contrast to the classical type, ACF(Min) were not elevated above the surrounding mucosa, their detection was totally dependent on methylene blue staining and transillumination, and they could not be identified with scanning electron microscopy. Histopathologic examination of ACF(Min) showed dysplastic crypts, similar to those found in larger lesions--that is, microadenomas in the Min mouse. The number of ACF(Min) increased up to the age of 6 weeks and then seemed to remain at a constant level of approximately four per colon. In conclusion, by transillumination of whole-mount preparations stained with methylene blue, we have identified and quantified small microscopic lesions that may be precursors of colonic adenomas in Min mice.