Pressure drops and collection efficiency have been determined for two Teflon-coated glass fiber filters: Pallflex T60A20 and TX40HI20-WW, and two Teflon membrane filters: Ghia Zefluor 2.0 and 3.0 μm pore size. The T60A20 filter gave the lowest pressure drops but had a clean filter collection efficiency of less than 80%. This efficiency rose quickly as the filter loaded, exceeding 95% for loadings greater than 0.03 mg cm-2. The other filters tested had efficiencies greater than 94% for all of the conditions tested, but had significantly higher pressure drops. The pressure drop produced by the collected particles was examined in some detail. The pressure drop divided by the product of face velocity and mass loading was found to be a constant which depended upon particle size and collection conditions. A relationship between this normalized pressure drop and particle size has been established. It may be used to estimate the mass mean diameter of diesel or diesel-like particles from relatively simple pressure drop measurements.