Forty-two patients with hepatitis Be antigen (HBeAg)-positive chronic hepatitis were treated by intramuscular injections with OK-432, an immunopotentiator possessing interferon-inducing activity. They were monitored with serial measurements of virological parameters to evaluate therapeutic effectiveness, and compared with a group of seventy-five untreated patients (natural course group). In the group receiving OK-432 therapy, twenty-seven patients (64.3% of the forty-two patients) became negative for HBeAg in an average observation period of 20.1 months. Of these, fourteen patients (33.3% of the forty-two patients) underwent seroconversion from HBeAg to anti-HBe antibody (anti-HBe). In the natural course group, twenty-three patients (30.7% of the seventy-five patients) lost HBeAg reactivity in a mean follow-up period of 32.3 months, and thirteen patients (17.3% of the seventy-five patients) became seroconverted. Thus, the drug group showed significantly higher percentages of patients with disappearance of HBeAg and seroconversion, notwithstanding the shorter duration of the follow-up. Young males and females, females generally, or patients with high serum GPT levels were more likely to respond to the therapy. The serum GPT level tended to stabilize more in patients receiving OK-432.