A set of procedures for the systematic separation and analysis of particle groups has been developed for use in a reexamination of early time aerial filter and local fallout samples of radioactive particles from nuclear detonations. The radioactive particles produced by nuclear detonations have radioisotopic compositions which vary with both particle size and type. To determine how individual radionuclides are distributed among the particles, it is first necessary to group the particles according to size and/or type. For the most part, conventional particle analysis techniques do not provide the required separated particle fractions. The range of particle diameters encountered extends from less than 0.01 microns up to several cm. No single separation technique is effective over this entire range. However, all samples fall into one of three narrower range categories for which single procedures do apply. Thus, for all local fallout samples, the majority of particles are in the 100-microns to few-cm diam range. Such samples are size separated by a modified dry sieving technique and density separated by flotation. All aerial filter samples from surface and near surface detonations have particles predominantly in the 1 to 100-microns diam range. These samples are size separated by gravitational sedimentation in stabilized fluid columns which are open at the base so that discrete size fractions can be collected according to time of sedimentation. Density separation is accomplished by density gradient centrifugation. The third category of samples, which includes all aerial filter samples from airbursts, contains particles whose size range is principally between 0.01 and 1.0-microns diam. These are subjected to size separation and density determination by centrifugal sedimentation in stabilized, open bottom, fluid columns.