The sampling accuracy of the fiber sampling cassette with a conductive inlet used in NIOSH Method 7400 was investigated, using a semiempirical model. The model, based on particle aerodynamic diameter, was used to calculate aspiration efficiency and inlet deposition. Experimental measurements from a wind tunnel and field measurements were compared with the results of the model. Effects due to particle inertia, gravitational settling, and vena-contracta deposition were included in the model. In the experiment, neutralized and charged fluorescein particles of 3 and 10 micrometers aerodynamic diameter were collected at chamber air velocities of 5, 30 and 100 centimeters/second (cm/sec) and air sampling velocities of 6.4 and 32cm/sec. The model agreed with the experimental data, within experimental error. Particles with large median diameters, such as nonasbestos fibers, may show increased aspiration biases and inlet deposition when sampling at low flow rates and large angles to the wind. The authors recommend sampling at the highest flow rate possible, and avoiding sampling where the air velocity is high. The angle between the inlet and the external air motion should be minimized. Conductive filters for filters with a conductive coating should be used. Cassettes should be tightly assembled before sampling and transported carefully after sampling.