Abstract: To investigate the relationship between 2', 5'-Oligoadenylate synthetase (2', 5'-OAS) activity in PBMC and interferon therapeutic efficacy in patients with chronic hepatitis B and so provide scientific basis for avoiding blindly useing interferon treatment in clinical therapy. 17 patients with chronic hepatitis B, both positives of HBeAg, HBV-DNA received interferon treatment first for a week and then 3 months to evaluate 2', 5'-OAS to the therapy. Among all the patients, 11 cases which were low in 2', 5'-OAS activities increased significantly after a week's treatment, (8 cases both positives of HBeAg, HBV-DNA returned negative; 3 cases only HBeAg returned negative) . There was no significant difference with the rest 6 cases a little high in 2', 5'-OAS before treatment and neither returned negative after treatment. So level of 2', 5'-OAS may be a useful marker to monitor interferon treatment efficacy during treatment to predict the clinical response.