Technical Abstract: The fungal toxin fumonisin B1 (FB1) occurs naturally in corn and other grains, and is a potential human carcinogen based on evidence of renal carcinogenicity in rats and hepatocarcinogenicity in mice in lifetime feeding studies. The toxicity and carcinogenicity of FB1 is linked to ceramide synthase inhibition, which leads to elevated levels of bioactive sphingolipids in blood and tissues. Based on this mechanism of action and on lack of evidence of genotoxicity, FB1 is considered a non-genotoxic carcinogen. The p53 heterozygous (p53+/-) mouse is a cancer-prone model used for carcinogenesis testing that has not previously been used to assess the toxicity of mycotoxin food contaminants. The effects of chronic dietary FB1 exposure were characterized in p53+/- mice to confirm non-genotoxicity using a model which is more sensitive to genotoxic than non-genotoxic carcinogens and to clarify the relationship between p53 expression, altered sphingolipid metabolism, and FB1-induced carcinogenesis. Responses to FB1 were similar in p53+/- and p53+/+ mice after 26 week exposure to 0, 5, 50 or 150 mg FB1/kg diet, supporting a non-genotoxic mechanism of FB1-induced tumorigenesis. The primary site of FB1 toxicity was the liver. Hepatic adenomas and cholangiomas were observed in p53+/- and p53+/+ mice exposed to 150 mg/kg FB1. For a 10% increase in hepatic megalocytosis, the estimated 95% lower confidence limit of the benchmark dose (BMDL10) ranged from 0.15 and 1.11 mg FB1/kg bw/day. Based on similar responses in p53+/- and p53+/+ mice, p53 and related pathways play a secondary role in responses to FB1 toxicity and carcinogenesis.