The method of the studying of the ionic content in the protein crystals by X-ray fluorescence spectroscopy is proposed. The ionic content of the tetragonal glutaraldehyde--cross-linked lysozyme crystals in the 2- 11 pH range and upon the low and high ionic strengths is studied. The acid-base titration of the lysozyme crystals is carried out for determination of the pH-dependence of the net charge of the lysozyme molecule within protein crystal. It has been shown that ionic content in the protein crystal channels is determined mainly by the Donnan potential. The specific binding of the bromide-ions with lysozyme molecules is revealed and possible binding sites are discussed.