We studied the anti-tumor effect of green tea polyphenol fraction (Sunphenon, SF: provided by Taiyo Kagaku Inc., Mie, Japan) on DMH-induced colorectal carcinogenesis in male Wistar rats. DMH was subcutaneously administered weekly at 20 mg/kg for 14 weeks. The rats in group I (20 rats) were given tap water for the whole of the study period. The rats in group II (15 rats) were given tap water from weeks 0-14, and 0.1% SF from weeks 15-35. The rats in group III (21 rats) were given 0.1% SF during the whole period. The rats were sacrificed at week 35. The cecal contents were aseptically removed and examined microbiologically to obtain the counts of four bacteria species (including Clostridium perfringens) per 1 g of cecal contents. The incidence of tumors production was significantly decreased (Group I: 100% vs Group II: 57.1%, Group III: 62.5%, p < 0.05), and the frequency of occurrence of C. perfringens (which is thought to yield harmful products which may be carcinogenic) was decreased in the SF-treated groups. These results suggest that SF prevents DMH-induced carcinogenesis in rats, and that its effect may be somehow related to its ability to preserve the composition of the colonic microflora.