further characterization of monoclonal antibodies to lipopolysaccharide of salmonella minnesota strain r595 we have shown here that despite the use of monoclonal antibodies with welldefined epitopespecificities and despite testing them in the most simple animal model available ie mixing of homologous lps with mab prior to injection we are not yet able to explain why some of the antibodies were effective and others not for some of the clones eg clone 20 an even better definition of binding sites is currently taking place in an attempt to obtain this understanding we also do not yet understand why clone 20 was not effective in the mucin model while using much lower amounts of injected antibody and much higher challenge doses this mab was effective against e coli in the gentamicintreated mouse model very clear is however that in order to be protective in the latter model mabs are not required to be specific for lipid a in the future it will be essential to develop procedures which measure specific interaction between smooth lpsbacteria and antibodies to the lps core region in addition it will be of great help when the chemical structure of nonsubstituted roughform lps as occurring in smooth lps preparations would be defined this applies also to osubstituted core molecules