Since chemicals may interact with cells in a variety of ways, the criteria for defining a toxic concentration depends on the technique used to measure toxicity. Thus toxicity can be expressed in terms of the doses of chemical which inhibit colony formation, cause detachment of cells from glass or plastic, inhibit DNA, RNA or protein synthesis, cause loss of metabolic activity etc. Where the toxic response is to be used as a parameter to relate to other effects such as mutation, transformation or DNA repair capacity, the method of measurement of toxic doses should be that most related to other end points. Thus, in determing the toxicity of chemicals to relate to mutagenicity the effect of a chemical in preventing colony formation is the most suitable criterion. However, in determining doses suitable for DNA repair assays, the doses resulting in detachment of cells from plastic is the most relevant criterion. Since the detachment of cells from plastic is the simplest and most rapid method it is routinely used to determine the approximate dose range in all experiments in In Vitro Toxicology. More accurate dose response data over narrower dose ranges can then be subsequently obtained.