This is a medium for determining the mutagenicity of a chemical reagent using the Ames Test. A histidine requiring strain (His- ) of Salmonella typhimurium is inoculated into a mixture of salt agar, histidine solution and the test reagent, which is mixed and then used to overlay the media. Following incubation if the test reagent is mutagenic it will reverse the His- phenotype allowing growth to occur at a higher level than the control.