Using the Praha strain of vaccinia virus (VV) two double recombinant VVs expressing the surface and capsid HBV proteins (HBsAg and HBcAg) under the control of the P7.5 promoter were constructed. In the first construct the gene coding for HBsAg was inserted into the HindIII J fragment (TK gene) and the gene coding for HBcAg was inserted into the HindIII M fragment (host range, K1L gene) of the VV genome. To test whether the expression of the foreign genes was influenced by the insertion site, in the second construct their locations were inversely changed. When compared with single VV-HBV recombinants expressing either HBsAg or HBcAg, the double recombinants expressed in vitro approximately the same amounts of the respective antigens. The particles formed by either HBsAg or HBcAg expressed by recombinant viruses, were isolated and examined by electron microscopy. Particles composed of both HBsAg and HBcAg were not detected in cultures infected with one of the double recombinants. The residual virulence in 3-week-old mice of the single recombinants was not markedly altered by the insertion of the second gene. The immunogenicity in mice of both the single and double recombinants was comparable and was not influenced by the location of the HBV genes in VV genome, as revealed by antibodies developed against the respective antigens.