Cell cultures of primary woodchuck hepatocytes can be infected with hepatitis delta virus (HDV) as demonstrated by the appearance of genomic HDV RNA 7 days after inoculation. This tissue culture system was used to study the effect of antiviral substances. Ribavirin inhibited HDV replication at a concentration of 10 micrograms/ml, if added up to three days post infection. Suramin had an inhibitory effect only when added simultaneously with the virus, at a concentration of 200 micrograms/ml. This concentration had no toxic effect on primary woodchuck hepatocytes. alpha-Amanitin showed a weak inhibitory effect only at the highest nontoxic concentration of 0.1 microgram/ml. Acyclovir had no effect.