text: One classical bioassay is the Ames test. A strain of Salmonella that requires histidine to grow is put on two plates with growth medium containing minimal amounts of histidine and some rat liver extract (to mimick liver metabolism). A suspected mutagen is added to one plate. If the plate with the suspected mutagen grows more visible colonies, it is probably mutagenic: a mutagen might cause the strain of bacterium to regain the ability to make its own histidine.
  Most other forms of toxicology testing are also bioassays. Animals or cell cultures may be put under a number of levels of a suspected toxin to ascertain whether the substance causes harmful changes and at what level it does so. The  value, a common measure of acute toxicity, describes the dose at which a substance is lethal to 50% of tested animals.
  The potency of a drug may be measured using a bioassay.