5-(Hydroxymethyl)furfural (HMF), one of the major intermediate products in the Maillard reaction, is present in a wide variety of foods. This aldehyde is formed as a decomposition product of glucose and fructose in foodstuffs subject to cooking or heat sterilization. It has been found to possess mutagenic and DNA strand-breaking activity. However, the mechanisms by which HMF exerts its genotoxicity remain unclear. The present study was undertaken to determine if HMF could be metabolically activated via esterification of the allylic hydroxyl group. In support of this concept, the chemically synthesized sulfuric acid ester,5-[(sulfooxy)-methyl]furfural (SMF), exhibited direct mutagenicity at both thymidine kinase and hypoxanthine-guanine phosphoribosyltransferase loci in human lymphoblasts. This reactive ester also induced 8-azaguanine-resistant mutants in Salmonella typhimurium TM677 in a dose-dependent manner. The intrinsic mutagenicity of SMF was enhanced by addition of extra chloride ion to the assay medium. The model allylic derivative, 5-(chloromethyl)furfural, was also mutagenic and cytotoxic in bacteria, but much more active than the sulfuric acid ester in this regard. In contrast to (sulfooxy)methyl and chloromethyl derivatives of HMF,2-[(sulfooxy)-methyl]- and 2-(chloromethyl)furans which lack the aldehyde functionality did not exhibit significant mutagenicity. Rodent hepatic cytosols contained sulfotransferase activity responsible for the formation of the reactive allylic sulfuric acid ester metabolite from HMF.