Nitrosatable precursors of mutagens that show mutagenicity to Salmonella typhimurium TA100 without S9 mix after treatment with nitrite at pH 3 were found in various foods. From Chinese cabbage, three indole compounds, indole-3-acetonitrile, 4-methoxyindole-3-acetonitrile, and 4-methoxyindole-3-aldehyde, were identified as mutagen precursors. 1-Methylindole and 2-methylindole, which are present in cigarette smoke showed strong mutagen precursor activity. Escherichia coli WP2 uvrA/pKM101 is more sensitive than S. typhimurium TA100 to nitrosatable precursors in soy sauce after treatment with 1-3 mM nitrite. The mutagenicity of soy sauce towards E. coli WP2 uvrA/pKM101 is partly explained by 1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid (MTCA) and tyramine reported previously. Oral administration of soy sauce and nitrite to male Fischer 344 rats for 2 years induced basal cell proliferation of the forestomach and intestinal metaplasia of the glandular stomach, but did not induce cancers in any organ. 3-Diazotyramine, a mutagenic nitrosation product of tyramine that is present at high concentrations in various foods induced squamous cell carcinomas of the oral cavity of rats when given in their drinking water. The carcinogenesis by N-benzylmethylamine, a nitrosatable precursor and nitrite was prevented by thioproline.