A placebo controlled clinical trial. Thirty two patients with chronic C hepatitis have been enrolled in a double blinded study to assess the therapeutic effect on an orally given antiviral-immunomodulatory drug, Isoprinosine. Seventeen patients were given Isoprinosine (3 g/day) and fifteen were on placebo. The treatment has been lasted for four months, when patients examined monthly. Clinical signs, liver function tests and side effects were evaluated. At the end of the trial, side effects and elevated serum alanine aminotransferase (ALT/GPT) levels occurred with higher frequency in Isoprinosine-treated patients. The results show that this antiviral drug has no beneficial effect in chronic C hepatitis.