The mutagenicity of a series of nine alpha-dicarbonyl compounds against S. typhimurium strain TA100 was studied using the Ames test (standard plate incorporation assay) without preincubation. Acetylbenzoyl, sodium glyoxylate and camphorquinone were not mutagenic. The following sequence of activities (in revertants per mumol of free dicarbonyl added) was obtained: glyoxal greater than methylglyoxal greater than phenylglyoxal much greater than 1,2-cyclohexanedione much greater than diacetyl greater than 3,4-hexanedione. These compounds can be grouped in three series: aldehydes, ketones and enolizable ketones (1,2-cyclohexanedione). In each of the two first groups the mutagenic activity decreases when the size of the substituent increases. No relation was found between the mutagenicity and the molecular electronic and/or resonance parameters. The low or non-existent activity of some of the chemicals studied is discussed. A relation between the mutagenic activities and the polarographic reduction potentials and, consequently, the structures of the mutagens was found.