Respirator filtration efficiency testing was discussed. The historical background of the development of federal research programs in respirator protection was reviewed. The Federal Government first became involved in testing respirators in 1910. With the exception of coal mining and a few other small industries, however, it was not until the Occupational Safety and Health Act of 1970 that using approved respirators became mandatory in general industry. The NIOSH respirator certification program was summarized. NIOSH and the Bureau of Mines have been authorized to test and certify respirators under Title 30 of the Code of Federal Regulations (CFR), Part 11. Current methods for evaluating particulate respirator filter efficiencies as mandated by Title 30 CFR Part 11 were outlined. NIOSH's research objectives for updating particulate respirator filter testing were described. These included determining worst case challenge concentrations, evaluating commercial particulate filters under worst case conditions, and comparing filter efficiencies as determined by the certification method with those obtained under worst case conditions. Current research on respirator filter efficiency being conducted at NIOSH was discussed. The studies have demonstrated the existence of a particle size at which a minimum efficiency occurs, which ranges from 0.02 to 0.29 microns. High efficiency (HE) filters showed minimum efficiencies at the largest particle sizes when compared to other filters. HE filters have shown efficiencies of more than 99.97 percent when tested under worst case conditions. Other filter types, such as dust and mist or paint lacquer and mist, not only had lower efficiencies but were not nearly as efficient as currently used testing methods indicated when they were tested under worst case conditions. Filter efficiency and particle size of minimum efficiency were dependent on flow rate. The author states that current certification tests do not indicate performance under worst case conditions.