Penetration through high-efficiency air filters was measured using dioctyl phthalate (DOP) particles and ambient particles. The tested filters were a Whatman GF/A filter and a Oshitari SO filter, one of the media used in high-efficiency particulate air (HEPA) filters. The particle concentration measurements were made with an automated diffusion battery (DB)/condensation nucleus counter (CNC) submicron-sizing system and a laser-particle spectrometer. It was found that the most penetrating particle sizes through both the GF/A filter and the SO filter were in a size range from 0.10 to 0.18 micron in diam. The values of penetration through the GF/A filter by two different particle-measuring methods, the combined CNC/DB system and the laser-particle spectrometer, were consistent with each other in the limited size range from 0.12 to 0.42 micron. No significant difference between the penetration curve of DOP particles and that of ambient particles was observed.