The formation of amyloid fibrils and other polypeptide aggregates depends strongly on the physico-chemical environment. One such factor affecting aggregation is the presence and concentration of salt ions. We have examined the effects of salt ions on the aggregation propensity of Alzheimer's Abeta(1-40) peptide and on the structure of the dissolved and of the fibrillar peptide. All salts examined promote aggregation strongly. The most pronounced effect is seen within the cationic series, i.e. for MgCl2. Evaluation of different possible explanations suggests that Abeta(1-40) aggregation depends on direct interaction between ions and Abeta(1-40) peptide, and correlates with ion-induced changes of the surface tension. Salts have profound effects on the fibril structure. In the presence of salts, fibrils are associated with smaller diameters, narrower crossover distances and lower amide I maxima. Since Abeta(1-40) aggregation responds to salts in a manner unlike that for other polypeptides, such as glucagon, beta2-microglobulin or alpha-synuclein; these data argue that there is no fully uniform way in which salts affect aggregation of different polypeptide chains. These observations are important for understanding and predicting aggregation on the basis of simple physico-chemical properties.