Peptide mass-fingerprint is widely used for protein identification while studying proteome with the use of 1D or 2D electrophoresis. Peptide mass tolerance indicates the fit of theoretical peptide mass with the experimental measurements, and choice of this parameter sufficiently influences the protein identification. The role of peptide mass tolerance was estimated by counting the number of identified proteins for the reference set of mass-spectra. The reference set of 400 Ultraflex (Bruker Daltonics, Germany) mass-spectra was obtained for the slices of 1D gel of liver microsomes. Using Mascot server for protein identification, the peptide mass tolerance value was varied in the range from 0.02 to 0.40 Da with a step 0.01 Da. Depending on the tolerance the number of identified protein changes up to 10 times. Maximal number of identified proteins was reported for the tolerance value of 0.15 Da (120 ppm), which is 1.5 - 2 times higher than the recommended values for such type of mass-spectrometers. The software program PMFScan was developed to obtain the dependence of number of identified proteins of the tolerance values.