AbstractUsing repair‐proficient and repair‐deficient strains ofE.coli, we investigated the application of a liquid incubation assay to measure the DNA‐damaging activity of ethanol‐soluble fecal extracts. This method appears to be suitable for the study of a wide range of sample types. It was used to measure the DNA‐modifying activity of ethanol‐soluble fecal extracts from a group of European colorectal cancer patients. Data were compared with those from Europeans of similar age and sex distribution who did not have bowel cancer. We also studied groups of Maoris, Samoans, and European Seventh‐Day Adventists who followed an ovo‐lacto vegetarian diet. There are significant levels of DNA‐modifying materials in the feces of many Europeans on a mixed diet, regardless of whether or not they have cancer. The number of positive samples was less in the Polynesian groups, and there were no samples that could be unequivocally scored as positive in the Seventh‐Day Adventist groups. We conclude that diet can significantly reduce the level of ethanol‐soluble mutagens, at least in New Zealand Europeans. The data may provide an explanation for the reduced incidence of bowel cancer in Seventh‐Day Adventist groups.