The relative affinities of various cations for anionic sites in isolated, bacterial cell walls were assessed by means of a technique involving displacement of one cation by another. The affinity series determined was H+ greater than La3+ greater than Cd2+ greater than Sr2+ greater than Ca2+ greater than Mg2+ greater than K+ greater than Na+ greater than Li+. High affinity was correlated with low mobility of the bound ions in an electric field. The net cation-exchange capacities of walls isolated from a variety of bacteria were estimated by preparing the magnesium forms of the walls, washing them well with deionized water to remove supernumerary ions, and then completely displacing the magnesium with Na+ or H+. Total amounts of magnesium displaced varied from 73 mumol per gram dry weight, for walls of the teichoic acid-deficient 52A5 strain of Staphylococcus aureus to about 520 mumol per gram for Bacillus megaterium KM walls....