Two isolates of Fusarium moniliforme from corn were used in a chronic study with groups of 30 inbred male BD IX rats fed a semipurified diet that was marginally adequate nutritionally. Group 1 served as the controls and received the semipurified diet containing 5% cornmeal, group 2 received 5% of strain MRC 1069 culture material that was nontoxic to rats, and group 3 received 0.5% of strain MRC 826 culture material that was highly toxic to rats. The amount of the mutagen fusarin C detected in the culture material of strains MRC 826 and MRC 1069 was 104 and 364 mg/kg, respectively. Survival up to 2 years was good in all groups. Pathologic examination showed that many rats in group 2 had mild ductular cell hyperplasia. Almost all rats in group 3 had neoplastic nodules, gamma-glutamyltransferase-positive foci, adenofibrosis, and esophageal basal cell hyperplasia. Whereas no tumors were induced in groups 1 and 2, the 21 long-term survivors in group 3 developed 8 cholangiocarcinomas, 2 hepatocellular carcinomas, 4 carcinomas of the forestomach epithelium, and 1 esophageal papilloma. Since neoplastic lesions were confined to rats in group 3 and the diet of these rats contained much less fusarin C than that of group 2, it is highly unlikely that fusarin C was responsible for the carcinogenicity of the MRC 826 culture material. It appears that the toxicity of F. moniliforme strains may be related to their carcinogenicity, but the chemical nature of the toxic and carcinogenic metabolite(s) produced by F. moniliforme MRC 826 remains unknown.