This is the first of two articles dealing with quantitative fit-testing of N95 filtering-facepiece respirators (a new class of particulate respirators in the United States). The purpose of this study was to develop a protocol for using the TSI Inc. PortaCount Plus to quantitatively fit test N95 respirators. The development of the protocol consisted of four parts: 1) designing a clamp so that the PortaCount Plus could be used to measure the penetration of ambient aerosol through N95 filters; 2) determining the magnitude of the ambient aerosol penetration; 3) comparing different filter penetration methods, i.e., the PortaCount Plus, a laboratory condensation nuclei counter (lab CNC), and the sodium chloride (NaCl) filter efficiency level test; 4) determining the effect of testing position on ambient aerosol filter penetration. Ambient aerosol filter penetration was measured with the PortaCount Plus and clamps designed in such a way that a small area of the filter was sealed so that the filter penetration of ambient aerosol through the sealed area was measured and used to estimate the penetration through the respirator when a wearer breathes. One-sided t-tests showed that filter penetration measured with the PortaCount Plus and a clamp was significantly greater than 0.03% (a=0.05). To investigate the effect of the filter penetration testing position, ambient aerosol filter penetration was measured with a clamp at 10 testing positions on five different respirators of the same model. Ambient aerosol filter penetration was found to be affected by the replacement of the clamp on the respirator. It was also found that the NaCl test provided filter penetrations that were significantly higher than those measured with the PortaCount Plus and lab CNC (a=0.05). To fit test N95 respirators using ambient aerosol, the filter penetration needed to be subtracted from the total inward leakage to obtain face-seal leakage. Finally, a protocol was developed to fit test N95 respirators, use the PortaCount Plus and the clamp to measure filter penetration, and adjust the measured fit factor for filter penetration. Part II gives the results of quantitatively fit-testing 21 N95 respirators using this protocol.