The mutagenicity of technical grade 2,4-dinitrotoluene (DNT), a mixture composed predominantly of 2,4-DNT (76.5%) and 2,6-DNT (18.8%) but containing 3,4-, 2,3-, 2,5- and 3,5-DNT, and the 6 isomers of DNT has been determined in 3 assays with Salmonella typhimurium. The mixture and the individual isomers of DNT were found to be mutagenic in the Ames' Salmonella/microsome test, particularly in strains responding to frame-shift mutagens; 3,5-DNT was found to be the most effective isomer in inducing reversion to histidine prototrophy in S. typhimurium strains TA98 and TA1538. Similar results were obtained with a quantitative reversion assay using strain TA98; 3,5-DNT was the most mutagenic of the compounds examined, particularly in the absence of post-mitochondrial supernants of rat-liver homogenates. While all isomers and the mixture of DNTs increased the 8-azaguanine-resistant fraction of S. typhimurium TM677 to some degree, the greater mutagenicity of 3,5-DNT compared to other isomers was not evident in the forward mutational assay.