The possibility of inducing an immune response to a protein expressed directly from an introduced gene represents an alternative to classic vaccination. We evaluated the ability of plasmid-based eukaryotic expression vectors to produce the Hepatitis B surface antigen (HBsAg) after injection of pure DNA into mouse tibialis anterior muscles. DNA was injected into either normal mature muscle, or regenerating muscle following cardiotoxin-induced degeneration. The sera obtained from these animals contained significant levels of HBsAg as early as 10 days after gene transfer, at which time low levels of antibodies to HBsAg (anti-HBsAg) were already present. Between 15-60 d after DNA transfer, serum levels of anti-HBsAg steadily increased whereas those for HbsAg fell, most likely due to the neutralizing effect of the antibodies. Analysis of proportions of HBs-seropositive mice showed that within 2 wk of injection of 100 micrograms pCMV-HBs in regenerating muscle, 91% of the mice were seropositive [defined as having more than 1 milli-International Unit/ml (mIU/ml) of anti-HBsAg]. Even at that early time, 68% had titers of anti-HBsAg greater than 10 mlU/ml, a level that is recognized as being sufficient in humans to confer protection against natural Hepatitis B virus infection. The proportion of seropositive animals rose to 95% by 4 wk, and 100% by 8 wk, at which time all mice had greater than 100 mIU anti-HBsAg in their sera. We have thus demonstrated that direct intramuscular injection of a plasmid vector encoding the HBsAg will give rise to secretion of the viral surface protein into the circulation which leads to an appropriate antibody response.(ABSTRACT TRUNCATED AT 250 WORDS)