The US Department of Health and Human Services has recently made available its Report on Carcinogens (RoC), as prepared by the National Toxicology Program located at the National Institute of Environmental Health Sciences (NIEHS). Users may view the entire report on this Web site provided by Environmental Health Perspectives, a peer-reviewed journal published by NIEHS. The RoC is an authoritative study that identifies and examines substances that pose a potential carcinogenic hazard to human health. For each listed substance, the report offers data on carcinogenicity, genotoxicity, biological mechanism, potential for human exposure, and existing Federal regulations to limit exposures.