We used the Ames method with the modification of pre-incubation to evaluate the potential mutagenicity of DMSO. We performed the assays using nine different Ames Salmonella strains and Escherichia coli strains WP2 and WP2uvrA. DMSO was found to be mutagenic for Salmonella typhimurium TA1537 and TA2637 (the latter strain being isogenic to TA1537 but carrying plasmid pKM101) and for E. coli WP2uvrA. The mutagenic activity of DMSO observed at a concentration of 33% was about 10 times higher than the background level (65 revertants induced) for TA1537 after 20 min of incubation, where some lethal toxicity was observed. The mutagenicity of DMSO was observed in the presence and absence of rat liver S9 mix.