The method previously described (Sinha, D. and Karush, F. (1979) Biochem. Biophys. Res. Commun. 90, 554–560) for the oriented attachment of immunoglobulins to lipid vesicles has been used to confer specific reactivity on liposomes by their conjugation with anti-lactose Fab′ fragments derived from rabbit IgG antibody. It is estimated that one-third of the Fab′ fragments was irreversibly attached to liposomal membrane, resulting in a membrane concentration of 2 mmol of Fab′ per mol of total lipid. The specific reactivity of the modified liposomes was demonstrated by agglutination with a multivalent, lactose-containing diheteroglycan. The availability of virtually all of the binding sites of the attached antibody for reaction with ligand was established by a fluorescence quenching titration with N-(N ϵ- Dnp- l- lysyl)-p- aminophenyl-β- lactoside . An intrinsic association constant of 8.9 · 10 6 M −1 was found for the attached Fab′ compared to a value of 2.8 · 10 6 M −1 for free anti-lactose Fab′. In addition the maximum values for the quenching by bound ligand of the fluorescence of free and attached antibody were the same. It can be concluded that the chemical procedures used to effect attachment of the antibody to the lipid vesicles allow retention of the original structure of the antibody site and its accessibility to external components.