In vitro test for mutagenicity using mutant strains of the bacterium Salmonella typhimurium which cannot grow in a given histidine-deficient medium: mutagens can cause reverse mutations which enable the bacterium to grow on the medium. The test can be carried out in the presence of a given microsomal fraction (S-9) from rat liver (see microsome) to allow metabolic transformation of mutagen precursors to active derivatives.