Francisella tularensis tularensis (Ft) is an intracellular Gram-negative bacterium and the causative agent of the severe human disease tularemia with potential for use as a bioweapon. Francisella lipid A, normally the biologically active component of lipopolysaccharide (LPS) has extremely low endotoxic activity. A Francisella tularensis novicida (Fn) lipid A biosynthesis mutant was generated that lacked the 4'-phosphatase enzyme (LpxF). Analysis of lipid A isolated from this mutant strain, compared to WT Fn showed retention of the phosphate moiety at the 4' position and the N-linked fatty acid at the 3' position on the diglucosamine backbone. This mutant was previously shown in our laboratory to be avirulent and confer protective immunity to a lethal WT Fn challenge. Further work has been carried out to elucidate the mechanisms of this protective immunity. The role of B cells was examined using μMT-/- mice, which lack functional B cells. While all mice survived the initial inoculation, all of the μMT-/- mice succumbed to the lethal challenge. This complete lack of protection shows that B cells are absolutely required for the protective response generated by the lpxF-null mutant. While development of an effective vaccine to Francisella remains a priority, we decided to address the therapeutic potential of serum obtained from immunization with the lpxF-null mutant in the possibility of a Francisella outbreak. Mice were infected with Fn and then were treated with serum either from naïve or immune mice. Mice receiving immune serum were completely rescued out to 36 hours post-infection, and were partially rescued at 48 hours, whereas mice receiving naïve serum started succumbing to infection at 60 hours post-infection, indicating the serum has therapeutic potential even late in an infection. The identity of the proteins recognized by this protective serum was further investigated. Our work not only identified known Francisella immunogens but revealed proteins previously unknown to be antigenic. Serum from LVS lpxF-null vaccinated-mice showed a similar protective capacity when given as a therapeutic. Current work is being carried out to generate monoclonal antibodies to these identified proteins and assay their ability to be used in the event of a Francisella outbreak.