Surface collection efficiency of membrane filters has been investigated covering a range of pore diameters from 0.5 to 10 micro. Measurements consisted of collecting natural airborne radon daughters on the filters and determining the resolution (FWHM) of the 7.68-MeV RaC' alpha activity using a solid state detector and multichannel PHA. Compared with submicron filters, large-pore membrane filters showed no serious sacrifice in the collection efficiency until the pore diameters exceeded 5 micro. Since these filters provide greater air flow at a given pressure differential than do submicron filters, use of larger-pore membranes may be better suited to sampling airborne alpha-emitting radioisotopes.