A DNA sequence involving pre-S2 region of Hepatitis B virus (HBV) was inserted between ompF and lacZ gene of the plasmid pORF1 to give a recombinant plasmid pHBSC20. In the plasmid pHBSC20 harboring E. coli TK1046 cells, a $quot;tribrid$quot; protein with the translation product of the insert sandwiched between OmpF and β-galactosidase was produced. The fusion protein was detected by SDS-polyacrylamide gel electrophoresis and the antigenicity of the pre-S2 region was determined by immunoblot analysis. The amount of the fusion protein was estimated as much as 1-596 of the total bacterial protein by densitometric scanning of the Coomassie blue-stained gel. Thus, bacteria transformed with pHBSC20 can provide abundant source of pre-S2-β-galactosidase fusion protein in which the pre-S2 region may be useful for the production of HBV vaccine.