We measured accurate NaCl main-term diffusion coefficients in aqueous lysozyme solutions at 25 degrees C and pH 4.5 using the Costing diffusiometer operated in its Rayleigh interferometric optical mode. The dependence of this diffusion coefficient on lysozyme concentration was examined using the obstruction-effect theory. Agreement between experimental results and theory is achieved if lysozyme proteins are treated as hydrated spheres with a hydration number of 240, a value that is comparable with those reported in literature for this protein. Electrostatic interactions and common-ion effects due to lysozyme net charge at pH 4.5 do not contribute significantly to the behavior of the NaCl diffusion coefficient within our experimental range of salt concentrations [(0.25 to 0.90) mol.dm(-3)].