Interferon alfa (IFN-alpha) treatment is effective in only a proportion of patients with chronic hepatitis B virus (HBV) infection. The mechanisms for therapeutic failure remain unknown but high levels of HBV replication are known to inhibit the immunopotentiating effects of IFN-alpha. In nine patients with chronic hepatitis B not responding to IFN-alpha monotherapy, we determined the virus-specific T-helper-cell responses during two consecutive therapeutic regimens: IFN-alpha alone and IFN-alpha in combination with a new potent inhibitor of HBV replication, lamivudine. By comparing the results obtained during the initial IFN-alpha monotherapy to those during the combination treatment, it was investigated whether complete inhibition of virus replication will enhance the interferon-induced immunoreactivity to HBV. Despite the rapid reduction to undetectable serum HBV DNA in all nine patients during the combination treatment, none sustained permanent hepatitis B e antigen (HBeAg) clearance during subsequent 12-month follow-up. HLA class II-restricted T-helper-cell responses to hepatitis B core antigen (HBcAg) showed no difference during IFN-alpha monotherapy and during the combination of lamivudine plus IFN-alpha. In contrast, a delayed T-cell activation occurred after a rebound in serum HBV DNA postcombination treatment, which lead to increased hepatocytolysis. These findings suggest that the profound inhibition of HBV replication by a nucleoside analogue does not restore the impaired virus-specific T-cell response in chronic HBV infection.