A new device has been developed for the measurement of the (212)Pb particle size distribution indoors. This device consists of two wire screens and a back-up filter with a diameter of 2.0 cm. The sampling flow rate is typically 3.0 l min(-1). After 3-h sampling time and 6-h waiting time, a CR-39 detector is used for the registration of the alpha particles from the (212)Pb, deposited on the wire screens and the filter, respectively. It appears clear from field measurements that there are no appreciable differences among the particle size distributions from different dwellings within the same location and under the same climate conditions. However, the (212)Pb particle size distributions from the countryside dwellings have different results from those of the city dwellings.