the program, had supplied the USSR with key information via outside contacts beginning in 1945. This information substantially advanced the Soviet effort in developing nuclear weapons. As the US became aware of the loss of key ideas and design concepts, it was important to reassess the state of the Soviet program in fission devices, and to monitor extensions of their program to fusion devices. One concern was the extent of progress made by the Soviets in producing fissionable material through reactor operations. Of even greater importance was the state of development of fission-fusion devices capable of orders of magnitude greater destructive power than the fission devices used against Japan. The detection of reactor operations and above-ground nuclear weapon tests can be carried out in various ways. One approach used since the early 1950s has been the analyses of air samples collected at ground and low-level altitudes for the presence of particular radioactive nuclides. This technique is still in use, and there are a number of ground-based, radionuclide detectors for airborne debris in continuous operation in various parts of the world. An advantage of collecting air samples in the stratosphere is that powerful above-ground nuclear explosions carry bomb debris to elevated altitudes, where it is broadly distributed by winds aloft. The collection and analyses of this debris can therefore provide direct and timely information about a particular test. This article briefly describes the successful effort to develop and implement a program of gathering air samples using high-altitude balloon systems that were used from 1953 until late in 1956. The high-altitude balloon program that collected these samples was known as Project Grab Bag. The Balloon and Meteorological Systems Group of General Mills, Inc. (GMI) had developed, along with other organizations, constant-level balloon systems capable of reliably carrying instruments and other equipment high into the stratosphere for sustained periods. These systems were ideal platforms on which to conduct experiments related to the observation of winds aloft, cosmic ray studies, and a variety of other investigations. Early in 1953 the eventual sponsors of the Grab Bag program conveyed to the GMI team under the direction of Harold E. Froehlich, principal engineer of the Balloons and Meteorological Systems Group, that high-altitude air samples containing debris from USSR above-ground nuclear weapon tests would be important in monitoring progress in their nuclear weapons program. The sponsors did not discuss specific kinds of debris or isotopes that were