[en] An extremely sensitive on-line monitor for alpha-emitting aerosols containing uranium and the transuranics has been developed. If the only alpha activity present were that associated with the decay of these aerosols, then monitoring for significant concentrations would be straightforward. There is, however, a naturally occurring source of alpha activity from the decay of radon and thoron daughter products. These background levels may be several orders of magnitude greater than the maximum permissible airborne concentration, MPC/sub a/, of the aerosol that is being monitored. The monitor that is described here incorporates discrimination against background activity that is based on two factors. Firstly, the background activity is associated with small-sized dust particles, while the aerosol to be monitored has most of its activity associated with large-sized particles. Secondly, the alpha energies from the two sources are different. The monitor utilizes virtual impactors to concentrate the large-sized particles into a flow that is a small fraction of the intake flow. The flow containing the large particles is then passed through a filter to collect the particles. The filter is viewed by a solid-state detector for alpha spectroscopy. The monitor is capable of detecting less than one MPC/sub a/ of soluble aerosols containing plutonium, 0.002 pCi/1, in less than one-half hour