Monoclonal antibodies that specifically agglutinate strains of Bordetella pertussis having serotype 1 agglutinogen were uniquely reactive with the electrophoretically slow-migrating A form of lipooligosaccharide. These monoclonal antibodies should be useful for the structural analysis of B. pertussis lipooligosaccharide and for the establishment of a better-defined serogroup for Bordetella species.