The interactions of N-acetylglucosamine (NAG), its α- and β-methyl glycosides, di- and tri-NAG with human and hen egg-white lysozymes [EC 3. 2. 1. 17] were studied by the circular dichroic (CD) technique. The association constants obtained for hen lysozyme were in good agreement with the results of other workers. The association constants of the saccharides to human lysozyme were smaller than those for hen lysozyme. The binding free energy of residue at each subsite of the substrate- binding site of human lysozyme was compared with that for hen lysozyme and was discussed on the basis of the recent results obtained by the X-ray crystallographic studies.