An analytical method was devised for the determination of the radon concentration in the air by measurement of the radioactive decay of the solid radon daughters collected on air filters during a fixed sampling period. The ratio of the radioactivity present at two periods, determined by an initial count immediately after termination of sampling and a second count an hour later, is used to estimate the extent of radioactive equilibrium established between radon and its daughters and to obtain a correction factor for relating radon daughter activity to radon activity. The disintegration rates of the various decay products of radon as a function of the age of the radon conglomerate have been calculated during the growth and collection periods and for successive 5-minute decay periods extending through the first 71 minutes following termination of filtration. A procedure for the calibration of Bata counters for the evaluation of RaB and RaC activities was established. Use was made of this analytical method to determine radon concentrations in the air near ground level. (Author).