A diet high in both fat and iron is known as a risk factor in cancer epidemiology. We found that lipid peroxyl radicals are generated from oxidized edible oils in the presence of heme-iron, which is present in high concentration in red meat, and they induce DMA damage including strand breakage and abasic site formation. The radicals are effectively scavenged by unpurified native vegetable oil containing flavonoids and other phenolic compounds. Although unpurified native vegetable oils contain a high amount of peroxyl radical scavengers, the conventional refining processes of edible oil seems to reduce the levels of such valuable components. The capability to suppress DNA breakage may indicate the preventive role of these components against fat-related carcinogenesis. In addition, peroxyl radical-scavenging activity parallels inhibitory effect on the tumor-promoting activity, as indicated by the "early antigen formation" in Epstein-Barr virus harboring Raji cells. This effect dramatically increases in hot-water-extracts of vegetables i.e. soup as compared to cold-water-extracts of fresh vegetables. These findings suggest that lipid peroxyl radicals, thus generated, may have a link between fat rich diet and colon carcinogenesis, involving DNA breakage, and tumor promoter effects, where red meat (heme) plays a crucial role. Therefore, the importance of peroxyl radical scavngers in crude plant oils and vegetable soup should be recognized in terms of cancer prevention and fat and aging-related diseases.