The influence of dietary selenium and cabbage on the formation of colon tumors in female Swiss mice treated with 1,2-dimethylhydrazine [(DMH) CAS: 540-73-8] was reported. Mice received a control diet (laboratory chow), the control diet plus selenium in the drinking water (1 mg/liter), or the control diet with added cabbage (12.8 g/100 g diet). They also received 8 weekly sc injections of DMH. The experiment was divided into two time periods: a) from 5 weeks before the first injection until 3 days after the last one (initiation period), and b) the subsequent 19.5 weeks until sacrifice of the mice (promotion period). Selenium had a strong protective effect when given during the initiation period; adenomas were reduced to a much greater extent than adenocarcinomas. The only effect of selenium supplementation in the promotion period was a small decrease in adenomas. Cabbage apparently had two opposing actions. It increased tumor incidence, particularly adenocarcinomas, if given in the initiation period, but it reduced adenoma formation considerably when given in the promotion period.