Biological membranes can be considered as two-dimensional fluids consisting of lipid molecules and membrane-associated polypeptides. Peripheral and trans-membrane polypeptides give rise to disturbances of the lipid bilayer. If the lipid-polypeptide interaction energy is not equal for the various lipid components of the membrane, the lipid mixture should be non-homogeneous. Furthermore, elastic deformations attributed to a local thickness alteration of the lipid bilayer can occur. Both considered effects, the local alteration of the lipid composition and elastic deformations, produce an indirect attractive force between adsorbed or embedded polypeptides. In many cases this attractive force turns out to be strong enough to enforce aggregation of membrane bound particles. (C) 2004 Elsevier B.V. All rights reserved.