The carcinogenic effect of carbazole (9H-carbazole) in (C57BL/6N X C3H/HeN)F1 (B6C3F1) mice was examined. Groups of 50 mice of both sexes were given a basal diet containing 0.6, 0.3, or 0.15% carbazole for 96 weeks and were then placed on an unsupplemented basal diet until they were killed at week 104. Mice surviving longer than 52 weeks were included in effective numbers. Significant increases in the induction of neoplastic lesions were found in the livers and forestomachs of mice given carbazole as compared to the control groups. The lesions in the livers were classified into two types, neoplastic nodules or hepatocellular carcinomas. The incidences of both types of lesions were almost 100% in mice treated with carbazole, and both types of lesions were significantly increased in all mice except the males given 0.6% carbazole in their diet. The incidences of hepatocellular carcinomas were in general higher than those of neoplastic nodules. Though not statistically significant, increased incidences of pulmonary metastases were found in the groups given carbazole. Significantly increased incidences of neoplastic lesions and papillomas in the forestomach in all groups of female and male mice were also noted in the mice fed 0.6% carbazole. Squamous cell carcinoma induction was significantly different in male mice given 0.6% carbazole as compared to controls. These results indicate that carbazole is carcinogenic to the liver and forestomach in B6C3F1 mice and suggest that carbazole may be an environmental carcinogen relevant to cancer risk in humans.