The effect of antisense oligodeoxyribonucleotides(oligo[dN]s) on hepatitis B virus(HBV) replication in HepG2 cells harboring a cloned HBV genome was examined. Antisense oligo(dN)s directed at translational initiation sites of S, pre C and P genes of HBV were treated to the cells and the amount of HBsAg and HBV DNA content were measured 72 hours after the treatment. HBsAg expressions in HepG2 cells harboring the HBV genome were inhibited 68%, 53%, and 46% by the treatment with antisense oligo[dN] directed at S, pre C, and P gene loci, respectively, and HBV DNA content in the cells was also reduced by the treatment of each antisense oligo[dN]. The doubling times of the cultured cells treated with 25 micrograms, 50 micrograms, and 100 micrograms of antisense oligo[dN]/ml medium were 43.3, 62.1, and 93.0 hours, respectively, compared with 37.5 hours of the untreated control cells. Cellular DNA synthesis was inhibited by the treatment with 100 micrograms/ml of antisense oligo [dN], however, no significant effect was observed by the treatment with 50 micrograms or less of antisense oligo[dN]/ml. These results suggested that antisense oligo[dN]s specific to the translational initiation sites of S, pre C, and P genes of HBV may have therapeutic potential for the suppression of HBV propagation in chronic HBV infected patients.