Mutagenesis testing program.

Until recently, mutagenicity testing was done on preselected compounds in a manner in which the testing laboratories knew the identity of the substances under test and the "expected" results, i.e., positive for compounds selected because of their carcinogenicity and negative for food additives. There is no completed study in which substances were tested blind using a standardized protocol. Also, little attention has been placed on reproducibility and variability within and between laboratories. These aspects are currently under investigation in microbial systems by NCI but the results will not be available 1-2 years. In the NIEHS testing program a large number of substances will be tested in a blind study. At present we suggest use of a short-term testing system consisting of microbial tests plus mammalian activation systems (Tier 1), two different Drosophila systems (Tier 2) and four different whole animal systems (Tier 3). The compounds will initially be screened for mutagenicity in Tier 1, and the results obtained in Tier 1 together with what is known about the compound otherwise will dictate the decision whether to continue the test of the compound in Tier 2 and Tier 3.