A novel recombinant interferon-alpha B/D hybrid was applied to assess tolerability, antiviral effect, and biological activity in chronic hepatitis B. The study was designed as an open nonrandomized trial. Treatment comprised a two-week run-in phase with 16 MU three times a week followed by 14 weeks with 64 MU three times a week (or 48 MU if toxicity occurred with 64 MU). Total follow-up was 36 weeks. Nineteen patients were included; three discontinued treatment during the run-in with 16 MU. Fourteen of 16 patients had 14 weeks of treatment with > or = 32 MU three times a week. Fourteen dose reductions were necessary in nine patients. The adverse experience profile was similar to other interferon-alphas. HBV-DNA decreased using all doses studied. HBV-DNA became undetectable in five patients, two of whom had HBeAg seroconversion. No HBsAg seroconversion was observed. It is concluded that interferon-alpha B/D is well tolerated in high doses. The anti-viral effect starts at at least 16 MU three times a week.