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The effects of ribosome-inactivating proteins (RIPs) from Ricinus communis and from Viscum album on the water permeability, P-f, and the surface dielectric constant, epsilon, of model membranes were studied. P-f was calculated from microelectrode measurements of the ion concentration distribution in the immediate vicinity of a planar membrane, and epsilon was obtained from the fluorescence of dansyl phosphatidylethanolamine incorporated into unilamellar vesicles. P-f and epsilon of fully saturated phosphatidylcholine membranes were affected only in the presence of a lectin receptor (monosialoganglioside, GM1) in the bilayer. It is suggested that the membrane area occupied by clustered lectin-receptor complexes is markedly less permeable to water. Protein binding to the receptor was not a prelude for hydrophobic lipid-protein interactions when the membranes were formed from a mixture of natural phospholipids with a high content of unsaturated fatty acids. These membranes, characterized by a high initial water permeability, were found to interact with the RIPs unspecifically, From a decrease of both P-f and epsilon it was concluded that not only water partitioning but also protein adsorption correlates with looser packing of polyunsaturated lipids at the lipid-water interface.