A series of antisense phosphorothioate oligodeoxynucleotides against hepatitis B virus (HBV) were synthesized and evaluated for their antiviral effect in Hep-G2 cells transfected with HBV genome. The inhibitory effect of the tested antisense oligonucleotides was sequence-specific, dose-and time-dependent, and synergistic for certain combinations. In virus-inhibitory concentrations the oligonucleotides were harmless to 2.2.15 cells. The most effective antisense oligonucleotides were found directed against the HBV mRNA transcribed from the cap site of SP II promoter, the portion of polyadenylation signal and the initiation region of gene S, with an inhibition of the HBsAg and HBeAg production by 85-95% and 50- 60%, respectively. To our surprise, antisense oligonucleotides directed against three key sites of HBV X gene blocked the expression of HBsAg, HBeAg and HBxAg. This fact might be related to the trans-activation of HBV X protein. Using radioisotope labelling, we demonstrated that Lipofectin promoted the cellular uptake and antiviral effect of antisense oligomers in 2.2.15 cells. These results suggest a therapeutic potential of antisense oligonucleotides in the treatment of patients chronically infected with HBV.