Mice immunised with lipopolysaccharide (LPS) from Francisella tularensis were protected against challenge with the live vaccine strain (LVS). However, when similarly immunised mice were challenged using the fully virulent F. tularensis strain Schu4, only an increase in the time to death was observed. Passive transfer of serum from LPS-immunised mice to naive mice afforded protection against F. tularensis LVS. LPS-immunised mice depleted of either CD4+ or CD8+ T-cells survived a F. tularensis LVS challenge although the rate of clearance of bacteria from the spleen was significantly reduced in the CD8+ depleted group. LPS-immunised mice boosted with F. tularensis LVS were re-challenged with F. tularensis Schu4. This cohort was significantly protected (LD(50) increased from <1 to >1000 CFU). However, passive transfer of serum did not confer protection and mice depleted of CD4+ or CD8+ T-cells did not survive.