Enzootic bovine leukosis (EBL) is a disease of cattle caused by the bovine leukaemia virus (BLV), a member of the family Retroviridae. Cattle may be infected at any age, including the embryonic stage. Most infections are subclinical, but a proportion of cattle (~30%) over 3 years old develop persistent lymphocytosis, and a smaller proportion develop lymphosarcomas (tumours) in various internal organs. Natural infection has also been recorded in water buffaloes and capybaras. Clinical signs, if present, depend on the organs affected and may include digestive disturbances, inappetence, weight loss, weakness or general debility and sometimes neurological manifestations. Superficial lymph nodes may be obviously enlarged and may be palpable under the skin and by rectal examination. Cattle with lymphosarcomas may die suddenly, or weeks or months after the onset of clinical signs dependent on the location and number of tumours and the tumour’s growth characteristics. Although several species can be infected by inoculation of the virus, natural infection occurs only in cattle (Bos taurus and Bos indicus), water buffaloes, and capybaras. Sheep are very susceptible to experimental inoculation and develop tumours more often at a younger age than cattle. A persistent antibody response can also be detected after experimental infection in deer, rabbits, rats, guinea-pigs, cats, dogs, sheep, rhesus monkeys, chimpanzees, antelopes, pigs, goats and buffaloes. No vaccine against BLV is available. BLV is not thought to be a hazard to humans.