Eight patients with chronic hepatitis B infection (seven with chronic active hepatitis and one with chronic persistent hepatitis) were treated with daily intramuscular injections of human leucocyte interferon for periods of 5 to 8 weeks and in one case for 5 months. In one patient there was a marked fall in virus-associated DNA polymerase activity and in the number of DNA containing viral particles during each of two courses of interferon. Hepatitis Be antigen (HBeAg) also disappeared, the aspartate transaminase levels fell and liver histology improved. In the four other patients with detectable DNA polymerase activity there was an early fall but this was transient and in one of these patients there was a continuing rise in activity despite treatment. One other patient became HBeAg negative but hepatitis B surface antigen (HBsAg) titres were mostly unaffected by treatment. A marked decrease in T-lymphocyte mediated cytotoxicity towards HBsAg coated target cells was demonstrated and raises the possibility that an immunosuppressant action of interferon may offsets its direct anti-viral action but may also account for the improvement in liver function which occurred in some patients.