Some of the widely used techniques employed in assessing asbestos load in lung tissue include the use of digestion techniques, in which particulates are entrapped on a filter surface. However, the actual filtering efficiency of various pore sizes as applicable to collecting fibrous material has not been tested. The present study evaluates such filtration efficiency by using a series of back-to-back filters of various sizes. It was confirmed that fibers pass through the pores and that with the larger pore sizes an appreciable loss of small fibers can occur. It is suggested that a filter with 0.2-micron pore size offered a reasonable compromise for both filtration efficiency as well as sufficiently rapid filtration rates for most studies.