Antisense oligodeoxynucleotides strategies have been used both to study normal gene function and to block gene expression therapeutically. We have previously shown that a number of antisense oligonucleotides against hepatitis B virus (HBV) mRNA are able to inhibit viral gene expression in vitro. Here we report the establishment of an animal model producing HBV markers in athymic nude mice and inhibition of HBV gene expression and replication by antisense DNA in vivo. 2.2.15 cells (Hep-G2 cell line transfected with HBV genomes) were injected subcutaneously (s.c.) into athymic BALB/c nude mice at a total cell number of 0.5-1 x 10(8) per mouse. Transplanted tumours developed about 2 weeks after inoculation. Hepatitis B surface and e antigens (HBsAg and HBeAg), as well as HBV DNA, could be detected in the circulation of tumour-bearing mice. Hepatitis B surface antigen and hepatitis B core antigen (HBcAg) were demonstrated in tumour cells. After 10 days of tumour growth, antisense phosphorothioate oligonucleotide, complementary to the cap site of the SP II promoter of HBV mRNA, were injected by infiltration into or around the tumour as a daily dose of 20 micrograms per gram body weight. Treatment for a total of 10 days resulted in an effective inhibition of viral replication and gene expression. These results suggest therapeutic potential for antisense oligomers in the treatment of patients who are chronically infected with HBV.