Carcinogenicity and chronic toxicity of biphenyl was examined in the male and female BDF₁ mice fed a diet containing biphenyl at 667, 2,000 or 6,000 ppm for 2 years. There was no difference in survival rate between any biphenyl-containing diet-fed group of either sex and the respective control. Body weights of the males and females fed 6,000 ppm diet were significantly lower than the respective control. Incidences of hepatocellular carcinomas and hepatocellular adenomas in the females fed diets containing biphenyl were significantly increased in a dose-related manner, and exceeded a range of the historical control data in the Japan Bioassay Research Center. Incidences of basophilic cell foci in the liver were increased in the females fed 2,000 and 6,000 ppm diets. There was no increase in tumor or tumor-related lesion in the males fed diets containing biphenyl. Chronic toxicity of biphenyl was characterized by increased incidences of urothelial desquamation in the renal pelvis in males and females and mineralization in the inner stripe of renal outer medulla in females, as well as changes in serum levels of BUN, ALP and some electrolytes in males and females. In conclusion, the 2-year oral administration of biphenyl-containing diets induced pre-neoplastic and neoplastic lesions in the liver of females and non-neoplastic lesions in the kidney of males and females. Causative factors for the biphenyl-induced hepatocarcinogenicity were discussed in light of our published finding of peroxisome proliferation.