We have used two bacterial proteins from Escherichia coli to express heterologous peptides. Both proteins are situated in the E. coli cell envelope but have different properties: LamB is an integral outer membrane protein, and MalE a soluble periplasmic protein. The peptides were expressed as genetic inserts within "permissive sites" of these recipient proteins, i.e. sites which allow the insertion of foreign peptides without affecting the biological properties of the host protein. In this paper, we summarize preliminary rules governing the immunogenicity of resulting LamB and MalE hybrid proteins when expressed in E. coli. We focus on two model epitopes: either peptide 132-145 from the preS(2) region of hepatitis B virus or peptide 93-103 from poliovirus VP1 capsid protein. We also present first results obtained when the same hybrid proteins were expressed in attenuated Salmonella typhimurium. Plasmids encoding the hybrid proteins were transferred to aroA S.typhimurium by electroporation. In vitro, the hybrid proteins could be expressed at high levels by S. typhimurium. Mice were immunized by parenteral and oral routes. The effect of the carrier protein and the level of its expression on the in vivo behaviour of the immunizing bacteria and on the immune response induced will be discussed.