Fourteen chimpanzees were inoculated with pre- and posttreatment sera from seven patients with persistent hepatitis B virus infection and chronic hepatitis who had permanent responses of their infection to treatment with interferon and/or adenine arabinoside. Inoculation of pretreatment serum at a dilution of 10−8 from a patient with a Type I response to treatment [disappearance of Dane particle DNA polymerase (DNAP) activity, HBeAg, and HBsAg from serum] resulted in infection, while undiluted posttreatment serum (all markers negative) failed to infect another animal. Pretreatment sera (DNAP, HBeAg, and HBsAg positive) from all of six patients with a Type II response to treatment (disappearance of DNAP activity and HBeAg but not HBsAg from serum) led to infection in six chimpanzees after inoculation of serum dilutions varying between 10−2 and 10−7. Inoculation of undiluted posttreatment sera (HBsAg positive and DNAP and HBeAg negative) from the same six patients produced no evidence of hepatitis B virus infection in another six animals. These results indicate that a Type I or II response to treatment with these antiviral agents reduces the infectivity in the serum of patients with chronic hepatitis B to below the level of detection by this assay. Such changes should be useful in interrupting spread of the infection between individuals. Our findings suggest that the serum of some patients who, without treatment are HBsAg positive and DNAP and HBeAg negative, may also be free of detectable infectious hepatitis B virus.