The clinical trials of alpha 2-interferon (alpha 2-IF) (reaferon) in 412 patients with acute viral hepatitis B (AVHB) permitted the establishment of the optimal therapeutic dose and the schedule for the drug use. The drug tolerance appeared satisfactory on the whole. alpha 2-IF was found to have a beneficial effect on the clinical course of VHB. The relationship was established between the therapeutic effectiveness of the drug and the clinical variants of the disease as well as the time of drug administration. The highest clinical effect (in 86.3%) was attained as a result of early (before the 7th day of jaundice) use of alpha 2-IF in patients with the acute-cyclic pattern of VHB. That effect consisted in the reduction of the treatment period (by 6-8 days), in a 2-fold reduction of the incidence of HBs-antigenemia, in rapid stimulation of the IF system, and in the initially low activity of natural killers. If the drug was administered later, the positive clinical effect was detectable only in 61% of the patients. In cholestatic VHB, alpha 2-IF did not produce any therapeutic effect.