Problems have been encountered in the repeatability of performance measurements in the panel filter universal test housing specified by the Society of Automotive Engineers' SAE J726 Air Cleaner Test Code. The flow fields inside the test housing were analyzed for two different sized filters using flow visualization and two component laser doppler velocimeter (LDV) measurements. The analysis clearly indicated that the flow inside the housing was very turbulent, recirculating and separated from the walls. The filter in the housing receives non-uniform velocities with the central part receiving higher velocities than the sides. Simple filtration theories were applied to calculate the local filtration efficiencies for monodisperse particles for a clean filter. The predicted local efficiencies were integrated to give the effect of the non-uniform velocities upon the overall filter efficiency. Calculations predict that for larger particles the filtration efficiencies with a uniform flow distribution are better than with non-uniform flow distributions. For small particles the filtration efficiency decreases for the uniform flow distribution.