Persistent infection with hepatitis B virus (HBV) is one of the primary risk factors for human hepatocellular carcinoma (HCC). In a human ecological study, we have shown that, in addition to HBV, animal food consumption also significantly contributes to the variance of HCC. To test the interacting effect of HBV and animal food consumption on the development of HCC, we investigated HBV expression in HBV transgenic mice fed three levels of casein diet. HBV expression in transgenic animals was substantially inhibited when dietary casein was reduced from the traditional level of 22% to the level of 6%. Northern analysis revealed that suppression of HBV was derived from both the upstream albumin promoter and the internal HBV promoter. Immunochemical staining of liver sections indicated that only a few hepatocytes around the central vein expressed viral surface antigen (HBsAg) in the 6% casein animals, whereas virtually all hepatocytes stained positively for HBsAg in the 22% dietary casein animals. Serum HBsAg concentrations at 4 months were increased by 1.6-, 2.1-, and 5.1- fold over baseline for animals fed the 6%, 14%, and 22% casein diets, respectively. Correspondingly, liver injury was much less severe in animals fed 6% casein diet than in those fed 14% and 22% casein diets. These results demonstrate that a low casein diet is a potent suppresser of HBV transgene and HBV-induced liver injury, suggesting that diet management may be a practical means to aid in the control HBV infection.