The promotion of 7,12-dimethylbenzanthracene (DMBA) induced mammary cancer in Wistar rats by a 4% cholestyramine (CHST) diet was investigated. The rats, 50 days of age, were divided into six groups. First two groups were given an intragastric dose of 0.8 ml of corn oil whereas the remaining four groups were given a single intragastric dose of 5 mg of DMBA dissolved in 0.8 ml of corn oil. After 1 week on laboratory chow the first two groups and two groups treated with DMBA were fed a control diet and the two remaining groups treated with DMBA were fed a 4% CHST diet. Half the animals were killed at 100 days and the remainder at 200 days. A detailed histologic examination of grossly normal mammary tissue as well as any tumour mass was made for each rat. The serum lipids were extracted and the individual neutral lipid composition was determined. In rats treated with DMBA and fed a 4% CHST diet, the incidence of malignant tumours increased by 5 fold, and the tumour weight by 12 fold. In addition, the serum total lipids, cholesterol esters and triglycerides decreased significantly when compared with rats fed a control diet. These results suggest that CHST diet promotes DMBA induced mammary cancers in Wistar rats.