Study of the efficiency of filters for the retention of natural airborne radioactive aerosols has shown that the paricles with which the radioactivity of the short-lived radon daughters is associated have greater ability to penetrate the variousfilter media than has the fission product debris in the atmosphere; therefore the former is associated with aerosols of smaller size. A preliminary evaluation of the techniques of employing packs of filters of different retentivity characteristics to determine the particle size and/or particle size distribution of radioactive aerosols has been made which indicates the feasibility of the method. It is recommended that a series of measurements be undertaken to determine the relative particle size distribution of RaB+C as a function of time, season and weather, and the size ditribution of fission products as a function of the above parameters and the source of the debris. (Author).