Abstract Distal ileal bypass or resection performed prior to the intragastric instillation of DMBA in sesame oil to 50–52-day-old female Holtzman rats significantly reduces tumor production. This effect is not present when animals are bypassed after the administration of carcinogen. Tissue levels of DMBA are low in the groups with lowered incidence of tumor and reflect decreased absorption of this agent. Significantly fewer tumors developed with ileal bypass than with ileal resection. The addition of cholic acid reverses this effect. Both serum cholesterol and weight gain are not affected by these procedures.