Live attenuated Salmonella vectors provide a unique alternative in terms of antigen presentation by acting as a vector for heterologous antigens. The efficiency of any live bacterial vector rests with its ability to present sufficient foreign antigen to the human or animal immune system to initiate the desirable protective immune response. Salmonella vectors encoding heterologous protective antigens can elicit the relevant immune responses, be it humoral, mucosal or cell-mediated. STM-1 is a Salmonella mutant developed by RMIT, harbours a mutation in the aroA gene that renders it attenuated, and is a well characterised vaccine strain currently in use to protect livestock against Salmonella infection. In previous work in this laboratory, STM1 was shown to be capable of eliciting immune responses in mice to plasmid-borne antigens. In this study STM-1 was analysed for its ability to vector the model antigen chicken ovalbumin and test antigen C. jejuni major outer membrane protein using in vivo inducible promoters such as pagC and nirB from the plasmid location. The determination of the architecture around the lesion in STM-1 also allowed the development of constructs expressing heterologous antigen from the chromosome. The induction of immune responses, both humoral and cell mediated, was analysed. Another issue addressed in this study was effect of pre-existing immune responses in the animal host against the vector or related strains and the effects on generation of immune responses against the subsequently vectored antigen. Humoral and cellular immune responses to vectored ovalbumin and C. jejuni Momp antigens were observed following vaccination with STM-1, when antigens were expressed from either the plasmid or chromosomal location. Up-regulation of immune responses, both humoral and cell mediated, was observed against the vectored antigens in animals which were pre-exposed to either the bacterial vector or related strains. These results indicate that STM-1 has the potential to be used as a vector to deliver heterologous vaccine antigens from a single copy gene in the field. Lastly, the results from this study indicate that pre-existing immune responses against the bacterial vector or a related strain do in fact enhance both humoral and T cell responses against the heterologous antigen.