||Mutagenicity is one of the important endpoints for risk assessment of environmental chemicals. Many short-term in vitro/in vivo assays to evaluate mutagenicity have been developed and some of them are being used routinely. Although these assays can generally be completed within a short period, their throughput is not enough to assess the large number of chemicals that exist in our environment without additional information on their safety. We have evaluated three commercially available in silico systems, i.e., DEREK, MultiCASE, and ADMEWorks, to assess mutagenicity of existing chemicals. We applied these systems to the 703 chemicals that had been evaluated by the Salmonella/microsome assay from the CGX database published by Kirkland et al. (2005). We also applied these systems to the 206 existing chemicals in Japan that were recently evaluated using the Salmonella/microsome assay under GLP compliance (ECJ database).