The interaction of membrane-bound ligand with bivalent and monovalent fragments of monoclonal antibody was studied by Fluorescence and precipitation analysis using synthetic lipid vesicles. The ligand Ne-[5-(dimethylamino)-naphthyl-l-sulfonyl]lysine was linked to the hydrophobic anchor dipalmitoylphosphatidylethanolamine and ranged between 0.01 and 1 mol % of the membrane components. The effects of cholesterol on the specific interaction were observed over the range of 0–50 mol %. A precipitation assay was developed to evaluate various factors related to the cross-linking of small unilamellar vesicles by bivalent antibody. The cholesterol content was critical for this process as demonstrated by the increased efficiency of precipitation over the range of 0–40 mol % of this component. Fluorescence analysis yielded the parallel finding of increased accessibility of the ligand to the antibody with greater cholesterol content. Increased surface density of the ligand also was found to enhance the intervesicle interaction. Finally, a comparison of the kinetics by fluorescence analysis of the binding of monovalent and bivalent fragments indicated that the bivalent interaction involved primarily the cross-linking of vesicles in accord with published findings of the interaction of monoclonal antibody with cell membrane antigens.