The Salmonella/microsome assay (Ames test) is one of the most widely used short-term tests. Despite the ubiquitous presence of this assay and the large number of chemicals tested, there is still controversy over the value of Salmonella/microsome assay results in risk assessment. Understanding of the properties and performance of the test system is necessary to provide an appropriate answer to this question. Based on both theoretical and empirical considerations, the results of the assay can provide useful information to assist in the development of further studies and as part of the data which can be used in evaluating potential biological effects or projected lack of adverse effects. The value of the Salmonella/microsome assay in performing these functions is considerably enhanced by consideration of all the information which the assay data provide.