The influence of eight different displacer salts on the retention properties of four globular proteins, ranging in molecular weight from 14 000 to 43 000, was investigated by using the Mono-Q strong-anion-exchange resin as the stationary phase. Proteins were eluted under gradient conditions with a range of alkali metal halides to vary systematically the anion and cation species in the series F-, Cl-, and Br- and Li+, Na+ and K+. Protein Zc values (i.e. slopes of the ion-exchange retention plots, as derived from the dependency of the logarithmic capacity factor log k′ on the concentration of the ionic displacer) generally increased when both the anion and cation were either chaotropic, e.g. KBr, or kosmotropic, e.g. NaF, in nature. Conversely, Zc values decreased when the displacer salt contained an anion-cation combination of a chaotropic and a kosmotropic ion, e.g. KF. These results indicate that the lyotropic properties of salts are additive in their effect on the interactive properties of proteins in anion-exchange chromatography. The Zc values were also found to depend on the manner in which the ionic strength was manipulated to affect elution, i.e. isocratic or gradient change in concentration of the displacing salt. Thus, isocratic experiments and gradient experiments with varied gradient time or varied flow-rate were observed to result in log k′ versus log 1/c dependencies with non-coincident Zc values. The relationship between protein Zc values, the electrostatic contact area or ionotope, Ac, and the electrostatic potential of the protein surface, ψs, is discussed.