The use of in vitro techniques in toxicological research is widespread, but, up to now, relatively little progress has been made in applying the knowledge gained in regulatory toxicity testing. In vitro tests should be accepted into regulatory toxicology for at least five reasons: scientific, humanitarian, legislative, logistical and economic. In particular, in vitro tests have the potential to provide a mechanistic basis for toxicity testing, and they may permit the use of tissues from more-appropriate target species and individuals, including humans. The relevance and reliability of the in vitro test, with regard to its use for a particular purpose and with particular types of chemicals, should have been adequately demonstrated (i.e. it should have been validated) prior to regulatory acceptance. There are several obstacles to this, including whether validation should be based on comparisons between in vitro data and animal data, and whether the in vitro tests should be expected to provide regulators with the same kinds of predictions and classification criteria that they currently obtain from animal tests. Regulatory incorporation should be a permissive process, rather than a restrictive one. Any scientifically defensible in vitro test which has been properly validated and independently recommended, should be acceptable for the specific purposes for which its use would be appropriate.