Eighty four patients with viral hepatitis attributed to infection with hepatitis B virus (HBV) (n = 43) or hepatitis C virus (HCV) (n = 41) were included in this study employing the MTH-68/B vaccine, an attenuated variant of Bursal Disease Virus. Twenty of the 43 patients in the HBV group, and 22 of the 41 HCV patients were treated with MTH-68/B. The remaining patients received conventional therapy. Significantly more patients progressed into active chronic hepatitis on conventional therapy (13% of HBV and 26% of HCV cases respectively) than in the vaccine treated groups (0% and 9%). Relapses occurred less frequently in the vaccine treated groups (5% of HBV and 32% of HCV) than in the control groups (9% and 79%), while remissions within one month of treatment were observed more often in the vaccine treated groups (both 50% respectively) than in the control groups (26% of HBV and 21% of HCV patients respectively). The duration of hepatitis was also considerably shortened by MTH-68/B treatment in both HBV (from 7.5 +/- 3.7 to 5.9 +/- 3.0 weeks) and HCV patient groups (from 8.9 +/- 7.4 to 5.3 +/- 4.4 weeks). The data presented suggest that attenuated, non-pathogenic viruses may be of significant benefit for patients with viral hepatitis B and C infections.