We have studied ion interactions of isolated cell walls from sunflower leaves, using cation exchange measurements, and microelectrophoresis. Ion exchange demonstrated that the cell wall binding sites had a strong preference for Ca2+ over Mg2+; but microelectrophoresis could not discriminate between these ions. The reason is probably that the latter method deals only with the most external charged sites of the wall, which have practically identical affinities for both divalent cations. On the contrary, the ion-exchange method has access to the internal sites, whose selectivity is much in favour of Ca2+. The discrepancy between the results of the two methods supports the hypothesis that the binding selectivity for Ca2+ is due to the spatial organization of the cell wall polyions rather than to intrinsic properties of their ionized sites.