Beta-interferon was administered by intravenous infusion to 16 patients affected with fulminant hepatitis B virus infection in third or fourth-grade coma. Ten patients presented a superinfection or a co-infection due to the delta (delta)-agent. None had detectable interferon (IFN) activity before therapy was begun. Besides fever, no significant side-effects were observed during treatment. Both the IFN-treated group as well as the "historical" control group, made up of 70 cases of fulminant virus hepatitis, not treated with IFN and observed during a previous ten year-period, received supportive therapy; survival rates were similar in both groups. Furthermore, the presence or absence of the delta-agent did not appear to affect survival rates significantly.