Lipid A derived from Gram-negative bacterial lipopolysaccharide is a potent adjuvant and antigen. Incorporation of lipid A into liposomes renders the liposomes themselves immunogenic, resulting in generation of specific antibodies that recognize either the individual liposomal lipids, or the unique pattern presented by the combination of lipids. Using liposomes containing lipid A, numerous polyclonal antisera and monoclonal antibodies have been produced against phospholipids, cholesterol, glycosphingolipids, and lipid A. Many of these antibodies have binding characteristics that are apparently similar to natural antibodies that are normally present in all human sera, and also antibodies that arise in response to various infections. Such antibodies probably represent a bridge between innate and adaptive immunity. The possible utility of liposomes containing lipid A as a constituent of certain types of novel vaccines was suggested by the observation that murine monoclonal antibodies to liposomal phosphatidylinositol-4-phosphate neutralized primary isolates of two different clades of HIV-1 in a human peripheral blood mononuclear cell neutralization assay.