Abstract
The 1980s have seen major research efforts focused on the development of alternative toxicity testing methodologies for the safety evaluation of chemicals. A difficult problem which has slowed the implementation of these methodologies has been validation. Validation means different things to different toxicologists; however, successful validation can take place only within the context of how the methodology is to be used: as a screen, an adjunct, or a replacement. The strategy for performing a validation study as well as the criteria for evaluation of the test methodologies will vary for each of these use categories. Correlative tests are adequate for screening purposes but mechanistic tests are required for adjuncts and replacements. The time has come to define the scientific basis of validation so that new methodologies can be efficiently processed through validation programs and incorporated into the safety evaluation process.