If a rogue nation tries to hide a nuclear test, a faint whiff of radioactive xenon leaking from the test site can unmask it. But a peaceful nuclear technology—the manufacture of medical isotopes—can produce almost identical emissions, confounding detection. Now, medical isotope–makers are pledging to help tame the problem; this month, the list of cooperating companies reached six. At stake is a worldwide network of hundreds of detectors—including 80 for radionuclides—that the Comprehensive Nuclear-Test-Ban Treaty Organization is setting up to sense any violations of the treaty. If plants' emissions mask the signal of an actual bomb, it will be much harder to verify an apparent violation and hold a country to account.