Induction of humoral and MHC (major histocompatibility complex)-I-restricted, cytotoxic T lymphocyte (CTL) responses of Balb/c mice to the small hepatitis B surface antigen (HBsAg) were studied with a protein antigen or a DNA vaccine. Different routes were used to deliver the HBsAg-encoding plasmid DNA or the recombinant HBsAg particles: different doses of expression plasmid DNA (10 micrograms or 100 micrograms per mouse) or of recombinant HBsAg lipoprotein particles were injected into different (normal or regenerating) muscles (m. tibialis anterior and m. quadriceps), into subcutaneous tissue (at the base of the tail), into the peritoneal cavity, or intravenously (into the tail vein). At different time points post-vaccination, the induction of HBsAg specific, MHC-I-restricted CD8+ T cells and of serum antibodies to HBsAg was monitored. The data show that the intramuscular and subcutaneous but not the intravenous and intraperitoneal injection of 'naked' DNA efficiently and reliably primes cellular and humoral immune responses. In contrast, recombinant HBsAg particles injected by all four routes (without adjuvants) efficiently primed specific humoral and CTL responses. These data demonstrate that the choice of routes to deliver 'naked' plasmid DNA for obtaining efficacious immunogenicity of the expressed antigen is restricted.