Mice fed a diet containing 0.3 or 0.03% triethanolamine developed malignant tumors. Females showed a high incidence of tumors in lymphoid tissues, while this type was absent in males. Tumors in other tissues were produced at a considerable rate in both sexes, but no hepatoma was found. Triethanolamine was not mutagenic to Bacillus subtilis by itself, but it became mutagenic after reacting with sodium nitrite under acidic conditions or when the mixture was heated. Although N-nitrosodiethanolamine, a known carcinogen and mutagen, was detected in the reaction mixture by thin-layer chromatography, it may not be the main mutagenic product, because the product was a stable and direct mutagen and its mutagenic activity was destroyed by liver enzymes, unlike N-nitrosodiethanolamine. The lethal and mutagenic DNA damages produced by this unidentified product were susceptible to some extent to the repair functions of the bacteria.