Stabilization of aqueous dispersions of highly hydrophobic colloidal particles-here three integral membrane proteins of nanometric dimensions-were obtained by substituting the classical protective layer of surfactant with low molecular weight amphiphilic polymers ("amphipols"). These polymers, derived from poly(acrylic acid), adsorb onto the particle despite the presence of residual surfactant from the initial stock solutions of proteins. Unbound amphipols and surfactants can be subsequently removed without dissociation of the complex or precipitation. As a consequence of multipoint attachment between the partners, complexation appears irreversible on the time scale of a few hours to a few days, Protein/ amphipol complexes contain a much lower number of alkyl groups per particle as compared with classical protein/surfactant complexes. The density of coverage (approximate to 100-200 alkyl groups per particle) increases with the salt concentration of the medium but is not sensitive to pH.