Protection against cholera has been correlated with the level of serum vibriocidal antibodies. The specificity of these vibriocidal antibodies was mostly to the lipopolysaccharide (LPS). We synthesized conjugates of detoxified LPS with cholera toxin (CT) and other proteins in order to elicit serum LPS antibodies with vibriocidal activity. Treatment with hydrazine (deacylated LPS) reduced the endotoxic properties of the LPS to clinically acceptable levels and resulted in a molecule larger and more antigenic than the saccharide produced by acid hydrolysis. More immunogenic conjugates resulted from multipoint compared with single-point attachment of the deacylated LPS to the protein. The conjugates containing CT had low levels of pyrogen and no toxic activity upon Chinese hamster ovary cells and elicited booster responses of vibriocidal and CT antibodies when injected subcutaneously as saline solutions into mice; the vibriocidal titers were similar to those elicited by comparable doses of cellular vaccines. We suggest how serum vibriocidal antibodies might prevent cholera.