Suramin, a polybasic anion, blocks the activity of the DNA polymerase encoded by the duck hepatitis B virus. The interaction with the virus was studied under conditions in which the property of the drug to bind with proteins was used with plasma obtained from infected ducks with viraemia. Inhibition of DNA polymerase activity associated with core particles from infected liver and circulating virions is irreversible, occurs in a dose-response fashion and suggests that inhibition results from suramin-virus interaction. The inhibition of the specific viral polymerase activity clearly indicates that this drug should be evaluated for the treatment of hepatitis B and infection caused by the related Hepadna viruses.