Nivalenol, a trichothecene, occurs widely in cereals and foods; our current two-year feeding trial has revealed no tumorigenic activity in female mice. To investigate whether dietary nivalenol modulates the development of aflatoxin B1 (AFB1)-initiated hepatocarcinogenesis, one-week old C57Bl/6 x C3H F1 mice were injected intraperitoneally with 6 mg/kg bw AFB1 and six weeks later fed diets containing 0, 6 or 12 ppm nivalenol for one year. Male mice in all three groups developed hepatocellular carcinomas and adenomas, while the incidences in females were 31% in those given AFB1 alone and 20% and 0 in those given AFB1 with 6 and 12 ppm nivalenol, respectively. These findings indicate that dietary nivalenol suppresses AFB1-initiated hepatocarcinogenesis in female mice, presumably by acting on the promotion step.