Twenty two hybridoma strains producing monoclonal antibodies against Francisella tularensis ATCC 6223, var. tularensis, were characterized. In an enzyme-linked-immunosorbent-assay (ELISA) using formaldehyde fixed bacteria as antigens, neither cross-reactions with six different Brucella spp., with Yersinia enterocolitica 0:9 nor with two biotypes of Yersinia pseudotuberculosis could be detected. The antibodies gave comparable titres with the three strains of F.tularensis tested. ELISA binding studies indicated that fifteen of the antibodies bound with high affinities to their epitopes of the three Francisella strains, while the others each seemed to bind with low affinity to at least one of the antigens. Immunoblot analysis showed that six of the antibodies were directed to epitopes on the core moiety of the lipopolysaccharide molecule, while the other 16 antibodies bound to O side chain components.