In order to assess whether polyadenylic.polyuridylic acid [poly(A).poly(U)] can be used as a new therapeutic agent for the treatment of chronic hepatitis B, 19 patients with histologically proven chronic active hepatitis B were injected intravenously with 100-150 mg of poly(A).poly(U) weekly for six weeks. Changes in alanine aminotransferase (ALT) levels, 2',5'-oligoadenylate synthetase (2'.5'-AS) activities and HBV markers were sequentially checked during and after treatments. Serum ALT levels were decreased gradually and 2'.5'-AS activities were significantly increased after initiation of poly(A).poly(U) injections. At the end of this trial (24th week) we have observed the normalizations of elevated ALT levels in 14 (73.7%), negative conversion of HBeAg in 11 (57.9%) and loss of HBV-DNA in 12 out of 19 patients (63.1%). Complete responses which had both normalization of ALT levels and negative conversion of HBeAg were noted in 11 patients (57.9%) and partial responses showing either normalization of ALT levels or negative conversion of HBeAg alone were in four out of 19 patients (21.1%). No notable adverse effects were observed during the treatments and follow-up period. It can be concluded that poly(A).poly(U) seems to be effective in the treatment of chronic active hepatitis B and has an advantage of being free of significant side effects.