Hepatitis B virus nucleocapsid/pre-S2 fusion proteins expressed in attenuated Salmonella for oral vaccination. Hybrid HBV nucleocapsid-pre-S(2) fusion proteins were stably expressed in several aromatic-dependent attenuated Salmonella typhimurium and Salmonella dublin strains. When these live recombinant bacteria were administered i.p. to BALB/c mice they induced high titer anti-hepatitis B virus core Ag (HBc) and detectable anti-pre-S2 serum antibodies. Upon oral feeding of the recombinant salmonellae to mice, the rate of seroconversion to anti-HBc was dependent on the salmonella strain used. With the best carrier strain high titer anti-HBc antibodies and lower titer anti-pre-S2 serum IgG antibodies were observed two weeks after a single oral immunization. The Ig class and IgG subclass distribution of anti-HBc antibodies after i.p. and oral immunization is consistent with the induction of functional T cell help. 