When the Toxic Substances Control Act (TSCA) was passed by the US Congress in 1976, its advocates pointed to new generation of genotoxicity tests as a way to systematically screen chemicals for carcinogenicity. However, in the end, TSCA did not require any new testing of commercial chemicals, including these rapid laboratory screens. In addition, although the Environmental Protection Agency was to make public data about the health effects of industrial chemicals, companies routinely used the agency’s obligation to protect confidential business information to prevent such disclosures. This paper traces the contested history of TSCA and its provisions for testing, from the circulation of the first draft bill in the Nixon administration through the debates over its implementation, which stretched into the Reagan administration. The paucity of publicly available health and environmental data concerning chemicals, I argue, was a by-product of the law and its execution, leading to a situation of institutionalized ignorance, the underside of regulatory knowledge.