The main objective of the present investigation was to understand the molecular events involved in the genesis of aberrant crypt foci. Aberrant crypt foci were induced in Sprague-Dawley rats with a single injection of azoxymethane. Aberrant crypts have been identified topographically in the colon and are hypothesized to represent preneoplastic lesions. In order to understand the molecular events involved in the early stages of colon cancer, PCR-amplified DNA from aberrant crypts was hybridized with oligonucleotide probes specific for the detection of point mutations in codon 12 of K-ras. The mutation identified was a G to A transition resulting in the substitution of the amino acid aspartic acid (asp) for glycine (gly). This mutation was present in 6/19 (32%) of aberrant crypts examined. The identical mutation was also identified in adenomacarcinoma tissue while no mutation could be detected in normal intestinal mucosa. For further confirmation of these results, the presence of the mutated ras protein (rasAsp-12) was detected in aberrant crypts by immunohistochemistry. This investigation provides the first identification of a ras point mutation in aberrant crypt foci.