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Persistent carriage of hepatitis B virus in extremely high titre was identified in 5 out of 9 chimpanzees kept at the London Zoo. Antibody to this virus was present in the other 4 chimpanzees. Serological survey of the other primates in the Regent's Park collection did not reveal the presence of the surface antigen in 2 gorillas, 11 orang-utans, and 2 gibbons, although surface antibody was present in the serum of 1 gorilla and 2 orang-utans. 3 of the carrier chimpanzees were born at the Zoo and were the offspring of either a carrier mother or a carrier father, and perinatal transmission may have occurred. A strict safety code of practice was introduced and hepatitis B immunoglobulin was given at intervals to designated staff members. Sero-conversion did not occur in any of the 38 staff members under surveillance for more than 2 years. Treatment of the carrier state in the chimpanzees was attempted with human leucocyte interferon, with and without ribavirin ('Virazole'), and with adenine arabinoside, but the effects were mostly temporary.