ABSTRACT - The general objective was to eatimate, from analytical data on cloud samples, the relative dis- tributton of certain radionuclides between the local and worldwide fallout formed by megatonrange detonations on land and water surfaces, with particular emphasis on the distribution of gr™ and Ca'3’ between local and woridwidefallout. It was planned to achieve these objectives by radiochemical analyses and particle size meas- urements on the following types of samples: (1) particles and radioactive gases present in the upper portions of the clouds to be collected by high-flying aircraft, (2) particulate matter in the clouds to be collected along nearly vertical flight paths, at several different distances from the cloud axis, by rocket-propelled sampling devices, and (3) faliout to be collected at an altitude of 1,000 feet by low-flying aircraft. shot The project participated in a 1.31-Mt shot (Koa) fired over a coralisland, a: in shalreef coral a over fired (Oak) shot 9-Mt a and water, deep in barge a from fired (Walnut) sets complete fairly and successful, generally was program sampling aircraft The low water. of both cloud and fallout samples were collected on each shot. The rocket program was unsuc- cessful because of a variety of equipment malfunctions. The gas samples were analyzed for radioactive krypton, and the cloud and fallout samples were each analyzed for Sr®, Cals! and several other nuclides to give information on fractionation. Fall rate and size distribution measurements were made on the particle samples from the land-surface shot. The combined analytical data was used to estimate the distribution of sr” and Cs'5" between the local and long-range fallout. There are no results to be reported on the spatial distribution of radioactivity in the clouds, because this part of the project was dependent on the rocket samples. The results from Shot Koa indicate that, if the cloud layers sampled were representative of their respective clouds, about one-fifth of the Sr™ and about two-thirds of the Cs‘*” produced were dispersed over distances greater than 4,000 miles. Corresponding fractions for Walnut were about one-third for each of the two nuclides. For Oak, the fractions were about one-third and one-half, respectively. Radionuclide fractionation waa pronounced in Koa and Oak, i.e., the radionuclide composition in the clouds varied with altitude. The local fallout was depleted, and the upper portions of the cloud were enriched in both Sr™ and Cs'’, Fractionation was much less evident in Walnut, the water-surface shot.