Abstract A novel method has been developed for the attachment of exogenous protein to liposomal and other membranes. A new phospholipid-containing alkylating reagent, N-(N α-idoacetyl, N ϵ-dansyl lysyl)-phosphatidylethanolamine, was synthesized for conjugation to sulfhydryl groups of proteins. Model experiments were carried out with a Bence-Jones dimer which was reduced to generate one sulfhydryl group per monomer. After alkylation with the reagent the modified protein spontaneously attached to preformed liposomes and red cell ghosts. The attachment and accessibility of the protein were demonstrated by the association of the protein with the liposomal fraction in gel filtration and the agglutination of treated vesicles and red cell ghosts with antisera to human λ chain and to the dansyl group.