Two identical, high-efficiency glass-media filter elements, rated β7(c) > 1,000, were tested using ISO and DFE methods. At a constant 112-lpm flow (ISO test) and a contaminant injection rate of 3 mg/liter, the filter maintained steady efficiency. But when flow cycled between 56 and 112 lpm (DFE test), particle counts after the filter element varied and were highest during changes from low to high flow.