From the 1960s to the 1980s, cancer was regarded by many scientists and government agencies as an environmental disease, one that could be controlled by regulating exposure to carcinogenic chemicals. I examine these ideas and ambitions from the lab bench up, by following the trajectory of an influential Petri dish test that was used to identify potential cancer-causing substances. The history of the Ames test, as it was called, provides a prism for viewing both the changing landscape of cancer biology and the struggle between environmentalists and industry over US chemicals regulation, in which testing requirements became a political battleground. By following a test rather than a law, I intend to extend work on the politics of regulatory decision-making to include materials and scientific practices. For example, several government agencies and international organizations launched programs involving technical experts to validate the Ames test against results from rodent carcinogenicity testing. Subsequently, criticisms of the standard rodent tests threatened to wobble the entire genotoxicity testing regime. In the end, while the Ames test became widely adopted in toxicology, its role in regulatory oversight of chemicals remained patchy and contested.