The Division of the National Toxicology Program (DNTP) within the National Institute of Environmental Health Sciences (NIEHS), is an interagency program whose mission is to evaluate agents of public health concern by developing and applying tools of modern toxicology and molecular biology. The program maintains an objective, science-based approach in dealing with critical issues in toxicology and is committed to using the best science available to prioritize, design, conduct, and interpret its studies. The evaluation of chemicals and other agents is generally conducted through a sequence of studies, which involve the exposure of laboratory rodents to these agents. Rats and mice are appropriate models for evaluating the toxicologic and carcinogenic potential of these compounds. The NTP utilizes the B6C3F1/N hybrid mouse to evaluate this potential. This contract supports the production of the NTP colony of B6C3F1/N mice.