AbstractSixty young adult Syrian hamsters were divided into five groups. Group 1 and Group 2 animals were treated with 0.25% dimethylbenz(a)anthracene (DMBA), painted on their left buccal pouches thrice weekly for 20 weeks. Starting at 12 weeks, at which time there was clinical evidence of leukoplakia and initial tumor formation, Group 2 animals received 10 mg retinyl acetate 3 times/week in a 5% solution in peanut oil, while Group 1 animals received only peanut oil. Two animals in Group 1 and two animals in Group 2 were sacrificed weekly from week 12 to week 20. Left buccal pouches were examined, tumors were counted, and tumor size was measured. Group 3 animals were untreated controls, Group 4 animals were retinyl acetate controls, and Group 5 animals received only peanut oil. It was found that DMBA‐treated animals receiving retinyl acetate from week 12 to week 20 developed fewer tumors, and their average tumor size was less than that in DMBA‐treated animals not receiving retinyl acetate. It appears that retinyl acetate, administered systemical‐ly, can retard tumor development even after leukoplakia has been established and tumors have begun to develop.