Fiber length is believed to be an important variable in determining various toxicological responses to asbestos and other elongate mineral particles. In this study we investigated screen collection characteristics using monodisperse-length glass fibers (i.e., 11, 15, 25, and 53 mu m in length), to better understand the collection of fibers with different lengths on screens with different mesh sizes. A well-dispersed aerosol of glass fibers (geometric mean length similar to 20 mu m), generated by vortex shaking, was fed directly into the Baron Fiber Length Classifier, in order to produce monodisperse length fibers. With nylon mesh screens (10, 20, 30, 41 and 60 mu m mesh sizes), the screen collection efficiency was measured using an aerodynamic particle sizer. As the screen mesh size decreases from 60 mu m to 10 mu m, the screen collection efficiency for 53 mu m fibers increases (from 0.3 to 0.9) while 11 mu m fibers exhibited a collection efficiency independent of screen mesh size. The collection efficiency for the longest fibers was found to be nearly constant for aerodynamic diameters 1-4 mu m for screens 20 and 30 mu m, but to rise significantly at aerodynamic diameters larger than 4 mu m. For the 20 mu m screen, the collection efficiency for fibers with lengths > 20 mu m is a factor of two to five larger than that for spherical particles with the same aerodynamic diameter. We believe that fibers are collected on the screen primarily by interception below 4 mu m in aerodynamic diameter, and by impaction above 4 mu m. This study represents a fundamental advance in the understanding of the interaction of screens with a fibrous aerosol.