Hydrogen-ion titrations were conducted for hen-egg-white lysozyme in solutions of potassium chloride over the range pH 2.5-11.5 and for ionic strengths to 2.0 M. The dependence of lysozyme's net proton charge, z(P), on pH and ionic strength in potassium chloride solution is measured. From the ionic-strength dependence of z(P), interactions of lysozyme with potassium and chloride ions are calculated using the molecular-thermodynamic theory of Fraaije and Lyklema.(1) Lysozyme interacts preferentially with up to 12 chloride ions at pH 2.5. The observed dependence of ion-protein interactions on pH and ionic strength is explained in terms of electric-double-layer theory. New experimental pK(a) data are reported for 11 amino acids in potassium chloride solutions of ionic strength to 3.0 M.