The modifying effects of dietary administration of 6-(2,5-dichlorophenyl)-2,4-diamino-1,3,5-triazine and 5 related compounds on the occurrence of azoxymethane (AOM)-induced colonic aberrant crypt foci (ACF) were investigated in rats. Male F344 rats were given s.c. injections of AOM (15 mg/kg body weight) once a week for 3 weeks to induce ACF. They also received the diet containing 200 ppm test compound for 5 weeks, starting one week before the first dosing of AOM. At the termination of experiment, all of the compounds had caused a significant reduction in ACF frequency, which might be associated with suppression of the expression of proliferation biomarkers. The apoptotic index in the colonic mucosal epithelium of rats killed at 6 h after the first AOM exposure revealed no blocking activity of the compounds.