Hepatitis delta virus (HDV) RNA editing controls the formation of hepatitis-delta-antigen-S and -L and therefore indirectly regulates HDV replication. Editing is thought to be catalysed by the adenosine deaminase acting on RNA1 (ADAR1) of which two different forms exist, interferon (IFN)-alpha-inducible ADAR1-L and constitutively expressed ADAR1-S. ADAR1-L is hypothesized to be a part of the innate cellular immune system, responsible for deaminating adenosines in viral dsRNAs. We examined the influence of both forms on HDV RNA editing in IFN-alpha-stimulated and unstimulated hepatoma cells. For gene silencing, an antisense oligodeoxyribonucleotide against a common sequence of both forms of ADAR1 and another one specific for ADAR1-L alone were used. IFN-alpha treatment of host cells led to approximately twofold increase of RNA editing compared with unstimulated controls. If ADAR1-L expression was inhibited, this substantial increase in editing could no longer be observed. In unstimulated cells, ADAR1-L suppression had only minor effects on editing. Inhibition of both forms of ADAR1 simultaneously led to a substantial decrease of edited RNA independently of IFN-alpha-stimulation. In conclusion, the two forms of ADAR1 are responsible almost alone for HDV editing. In unstimulated cells, ADAR1-S is the main editing activity. The increase of edited RNA under IFN-alpha-stimulation is because of induction of ADAR1-L, showing for the first time that this IFN-inducible protein is involved in the base modification of replicating HDV RNA. Thus, induction of ADAR1-L may at least partially cause the antiviral effect of IFN-alpha in natural immune response to HDV as well as in case of therapeutic administration of IFN.