AbstractTwenty‐five Lewis‐Wistar rats, weighing 500 to 525 gm each, received a transplantable mammary tumor which spontaneously arose in this strain; two days later, they were equipped with a silastic catheter inserted into the superior vena cava, the catheter protected by a specially designed harness allowing unrestricted movement of the animals.On the fifth day after tumor implantation, continuous intravenous infusions were begun, five rats serving as control and receiving only saline, twelve rats receiving glucose and eight rats receiving fucose.All 25 animals tolerated the procedure with no toxicity in any group as determined by animal inspection, measurement of blood urea nitrogen and of serum glutamic pyruvate transaminase. There was no significant weight change during the course of the experiment.On completion of infusion, tumors in rats receiving 20% fucose showed a statistically significant smaller mean diameter as compared to both controls and rats receiving intravenous glucose. After cessation of the ten‐day infusion the tumors in the fucose‐treated rats remained significantly smaller than those in their glucose counterparts or controls. Attendant on intravenous fucose therapy was a definite increase in total oral intake and urinary output. Twenty‐one days after conclusion of intravenous therapy all animals were sacrificed, and the tissues of those who received fucose were found to have a statistically significantly increased level of serum protein‐b