Invasive intracellular bacteria are able to transfer eukaryotic expression plasmids into mammalian host cells in vitro and in vivo. This can be used to induce immune responses toward protein antigens encoded by the plasmid or to complement genetic defects. Plasmid transfer takes place when the recombinant bacterium dies within the host cell, either due to metabolic attenuation or induction of autolysis. Alternatively, antibiotics can be used and spontaneous transfer has also been observed, indicating that this phenomenon might also occur under physiological conditions. Plasmid transfer has been reported for Shigella flexneri, Salmonella typhimurium and S. typhi, Listeria monocytogenes and recombinant Escherichia coli, but other invasive bacteria should also share this property. In vivo attempts were mainly directed toward vaccination using shigella and salmonella as carrier. So far a wide variety of antigens have been used succesfully in mice. Often this type of immunization was superior over direct application of antigen or using the same bacterium as a heterologous carrier expressing the antigen via a prokaryotic promoter. Characterization of the host cells revealed that macrophages and dendritic cells might be responsible for immune stimulation by either expressing the antigen or cross-presenting the antigen after uptake of apoptotic antigen expressing cells.