The immune response against hepatitis B surface and core antigens was evaluated by either coinoculation or independent intramuscular administration of pAEC compact DNA immunization vectors carrying their genes. The pAEC vectors bear just the essential elements for mammalian expression and bacterial amplification. Balb/c mice were immunized with 100 mug of each construct, either alone or in combination. In spite of lacking known immunostimulatory sequences (e.g., AACGTT), significant cellular (proliferative) and humoral immune responses were raised against both antigens. Coadministration of both plasmids maintained the immune response against the two antigens, without interference between them. Modulation of the antigen expression and further immune response, by using the Kozak's translation initiation sequence, was also analyzed. No differences due to its presence or absence were observed.