Virgin female C3H/He mice (275) were fed a ration that included 0.1% N-4-(5-nitro-2-furyl)-2-thiazoyl formamide (FANFT) [a carcinogen] and retinyl acetate. The mice were divided into 5 groups with groups 1-4 receiving 300, 600, 1200 and 2400 IU of retinyl acetate/kg of feed plus FANFT respectively. Group 5 received 300 IU/kg of feed and no FANFT as a control. After 45 wk the bladders were removed and inspected for neoplasms. There were no tumors found in any of the mice from group 5. Retinyl acetate significantly inhibited the formation of squamous cell tumors at all levels and inhibited transitional cell carinoma when given as 600 IU/kg of feed.