Preliminary experiments employing the Mudlux reporter system indicated that the ahp locus was osmotically sensitive in S. typhimurium. Studies on other bacteria supported this view. The affect of the osmotic environment of the cell upon the expression of ahp was therefore addressed in greater depth. The subsequent use of immunoblotting techniques conclusively demonstrated that chromosomal expression of the ahp locus was not affected by the osmotic environment surrounding the cell. Instead, the Mudlux element was found to alter the natural behaviour of the ahp promoter in such a way that it adopted an osmotically-regulated status, and this mode of regulation appeared to override regulation via the normal hydrogen peroxide-inducible mechanism. S. enterica is an intracellular pathogen which is capable of surviving within macrophage cells. Macrophages are equipped with an arsenal of anti-microbial effector mechanisms, including a respiratory burst which generates reactive oxygen metabolites. Since ahp had previously been shown to respond to the respiratory burst of macrophages, this study also assessed the role of oxidative stress resistance genes in the virulence of S. typhimurium. Strains of the mouse pathogen Sl1344 were constructed in which the ahp and oxyR loci were disrupted and their virulence was assessed in LD₅₀ studies. Disruption of the ahp or oxyR loci was found to have no effect upon the gross virulence of SL1344 for mice, suggesting that these loci were not essential for survival within the macrophage. The ability to develop immunity against infection by S. typhimurium is thought to correlate with the development of immunity against bacterial antigens which are expressed in vivo. As a further part to this study, the immunological responses of mice to two S. typhimurium-derived polypeptides, AhpC and GroEL, following cloning and overexpression of these proteins, were examined. Mice previously infected with an attenuated strain of S. typhimurium were shown to elicit significant delayed-type hypersensitivity reactions following subcutaneous injection of these polypeptides 33 and 104 days post-infection. Moreover, AhpC-and GroEL-specific antibodies were detected during the course of infection of mice with S. typhimurium.