° 2) 0012933 the fraction of the volume of the cloud swept out in obtaining each - sample 3) the mass of each of the two groups of particles in each sample 4) the R values of the radionuclide of interest in each of the two groups of particles in each sample The first of these can be calculated in advance from the criteria for world- wide fallout from the altitude of sample collection. The second can be calcula- ted from the area of the sampling system by obtaining the total volume of the cloud and the cloud dimensions at various altitudes from cloud photography. The third can be obtained by separating the particles into the necessary two fractions during sampling and subsequently weighing each group. The fourth can be obtained by radiochemical analyses of each of the two particle groups. It is recommended that such a program be carried out if the opportunity is presented by future weapons tests.