A genetic procedure has been previously established to expose a foreign epitope at the surface of Escherichia coli by using the outer membrane LamB protein as a carrier. A portion of the pre-S2 region of hepatitis B virus, residues 132-145, has been inserted at amino acid position 153 of the LamB protein, in a cell surface exposed loop. In the present study, we have analysed the antibody responses induced by these recombinant bacteria (live, heat-killed or sonicated) depending upon the route of immunization. The intravenous (i.v.) or intraperitoneal (i.p.) administration of the live recombinant bacteria to mice induced the synthesis of antibodies against both the inserted peptide and the native LamB protein. The antibodies raised recognized HBsAg particles. These mice also had high titres of antibodies against E. coli antigens (as determined using a crude bacterial sonicate). In contrast, mice immunized subcutaneously (s.c.) did not develop antibodies against the pre-S2 peptide nor against the HBsAg particles. Their anti-LamB responses were low compared with the response of mice immunized by the parenteral route. Interestingly, s.c. or i.v. immunizations induced comparable levels of anti-E. coli antibodies. Thus, the antibody response to the inserted peptide generally parallels the response to the LamB protein (and not to the bulk of E. coli antigens). However, this treatment corresponding to a 'pre-processing' of the recombinant bacteria was not sufficient to obtain an anti-peptide response following s.c. immunization.