The objective of this contract is to provide methods development and testing capacity that will permit the Cellular and Genetic Toxicology Branch to continue its improvement, assessment and application of short term, in vivo tests for genetic toxicity. This award will include projects involving tests for micronuclei, chromosomal aberrations and sister- chromatoid exchanges in mice and the development of testing capabilities in rats. Tests are conducted using, primarily, the bone marrow cells. Test results from these studies are used in the process of selecting chemicals for long term toxicity tests, by the Toxicology Research and Design Committee, in the overall evaluation of long term toxicity studies, and in the selection of chemicals for germ cell mutagenicity studies.