Researchers at the National Animal Disease Laboratory have developed a procedure for evaluating high-efficiency filters mounted in exhaust ducts. To test the filters, the aerosol organisms Escherichia coli B T3 bacteriophage, was generated in a chamber attached to to a ceiling exhaust register in concentration of at least 1000 organisms per three feet of air. Samples were collected from the pre- and post-filter areas, and the number of organisms per three feet was determined. The efficiency of the filter was calculated from the 269 filters tested. Of the 269, 249 had efficiencies of 98% or better. The remaining 20 were repaired and retested. No filter was accepted with an efficiency below 98%.