This article explores the Eisenhower administration's efforts during 1960 to tackle the apparent nuclear-proliferation risk posed by innovations in gas-centrifuge technology. Washington developed a policy of denial, first tried out in 1954 when Brazil tried to purchase gas centrifuges in West Germany. In 1960, with advances in gas-centrifuge technology raising the possibility of secret uranium-enrichment plants, Atomic Energy Commission and State Department officials agreed that it should be classified secret to limit worldwide access. Yet West Germany, the Netherlands, and the United Kingdom were already undertaking gas-centrifuge research and development and German research was unclassified. While secrecy for any West German work on nuclear technology raised difficult political questions, without German co-operation security classification for gas-centrifuge technology in the United States and the other countries could fail. As a holding action to check nuclear proliferation, US diplomats reached agreement with the Germans, Dutch, and British on classification standards. Washington also sought export controls to limit access to the technology. Leaks to the press complicated diplomacy, but the four-power understanding on gas-centrifuge secrecy lasted for years, although A. Q. Khan significantly undermined it when he purloined Dutch centrifuge technology in 1975.