Rat colon carcinogenesis induced by 2 x 15 mg/kg body weight of azoxymethane (AOM) is a standard model, widely used to evaluate the role of nutritional components and chemopreventive agents at various stages of tumorigenesis. In this model, KRAS mutations have been frequently observed in aberrant crypt foci (ACF), putative preneoplastic lesions, as well as in tumours. Therefore we used this model and vaccinated F344 rats with a mixture of synthetic mutant KRAS peptides (MT KRAS) corresponding to frequent KRAS exon 1 mutations before AOM treatment in order to study the role of KRAS mutations in the development of ACF and subsequently tumours. The controls were sham-vaccinated with KRAS exon 1 wild-type KRAS peptides (WT KRAS). MT KRAS vaccination suppressed the number of ACF by 42% at week 13 (p=0.001). The subpopulation of ACF suppressed by MT KRAS vaccination had higher focal crypt multiplicity than the control ACF population (p=0.001). At week 26, vaccination reduced the KRAS mutation frequency in ACF from 50% in the MT KRAS group to 13% in the WT KRAS (p=0.038). However, at this phase of carcinogenesis, vaccination did not have significant effects on the ACF number and focal crypt multiplicity. Surprisingly, the KRAS mutation frequency was only 5% in the colonic tumours of the controls (1 out of 20 tumours). Although there were no tumours with KRAS mutations in the MT KRAS group, the possible effect of vaccination could not be evaluated. These data indicate that KRAS mutations play a minor role in colonic tumorigenesis and that ACF with KRAS mutations could hardly be the precursors of the AOM-induced tumours in rats. Hence, the cancer protective potential of a KRAS vaccine in the early phase of AOM-induced colon carcinogenesis in the rat appeared minuscule. Additional studies in a model with a high outcome of KRAS mutations in colonic tumours are needed to evaluate the effects of a KRAS vaccine at later stages of tumorigenesis.