The effect of diet supplemented with 5% and 15% cellulose or with 15% fiber isolated from red beet (Beta vulgaris var. rubra) on the development of alimentary hypercholesterolemia and chemically induced colon carcinoma was studied in male Wistar rats. Hypercholesterolemia was induced by a diet containing 0.3% of cholesterol and colon carcinoma was induced by treatment with dimethylhydrazine (20 mg/kg, 12 doses applied s.c. in one-week intervals). Fibrous matter isolated from red beet contained 89% fiber, of which 9% was in water soluble form. Animals were killed 14 weeks after the application of dimethylhydrazine (i.e. 26 weeks after starting on the diets). Red beet fiber diet (and not the increased cellulose intake) caused a reduction of serum cholesterol and triacylglycerol levels (by 30 and 40%, respectively) and a significant increase in the fraction of cholesterol carried in HDL. This diet induced also a significant decrease (almost by 30%) of cholesterol content in aorta. Higher cellulose content in the diet and even more so the administration of red beet fiber caused a significant reduction of conjugated dienes content in plasma, erythrocytes and in liver. Also observed were increases in the activities of superoxide dismutase and catalase in erythrocytes and in colon and activities of glutathione peroxidase and glutathione-S-transferase in liver. The presence of both higher cellulose content and red beet fiber in the diet significantly reduced the incidence of precancerous lesions--aberrant crypt foci--in the colon. The diet containing red beet fiber did not affect significantly the incidence of colon tumours although the number of animals bearing tumours was reduced by 30%.