Chimpanzees chronically infected with hepatitis-B virus showed transient changes in several markers of infection when treated with the interferon inducer polyriboinosinic-polyribocytidylic acid-poly-l-lysine carboxymethyl cellulose. Serum Dane-particle-associated D.N.A. polymerase, e antigen and hepatitis-B surface antigen, and intrahepatic hepatitis-B surface and core antigens diminished during treatment. Defective (D.N.A.-polymerase-negative) Dane particles increased in titre transiently during treatment; these may play a role in the modulation of hepatitis-B virus infection. Humoral immune responses in chronic hepatitis-B carrier chimps were unaffected. Interferon inducers (or exogenous interferon) may be useful for the treatment of chronic hepatitis-B virus infection.