Feline leukaemia virus (FeLV) usually occurs in its natural species, the domestic cat. FeLV is also important to human individuals as a comparative model, as FeLV may cause a variety of diseases which are partly malignant and partly benign, such as immunosuppression which bears a resemblance to AIDS (acquired immune deficiency syndrome) in man. Although FeLV is a common infective agent, the incidence of disease due to FeLV is much higher in cats kept in closed households in which several of them are present than it is in free-range cats. Consequently, diseases caused by FeLV are frequently diagnosed in pedigree cats which are often maintained in relatively large numbers. FeLV is transmitted among cats by contagion. The main sources of infection are persistantly infected FeLV carrier cats which continuously excrete virus from the mouth and in other secretions. The majority of cats infected with FeLV will produce neutralising antibodies. Cats which are unable to do so, will become permanently infected. The prognosis is bad in these cats: 90 per cent will die within five years. Various techniques are used to detect FeLV. The most common method, the indirect immunofluorescence (IFA) test, is performed on air-dried blood smears. The results of the IFA agree with that are almost completely identical to those of the virus isolation test. Another test is ELISA (enzyme-linked immunosorbent assay), which produces approximately 10 per cent more positive results which are probably due to circulating FeLV antigens. Dissemination of FeLV among cats may be prevented by identifying infected carrier cats and removing them from contact with non-infected cats. Removal programmes using indirect immunofluorescent antibody (IFA) tests were used successfully in the Netherlands. The proportion of FeLV-positive cats decreased from 9 per cent in 1974 to approximately 3 per cent in 1985 during such a removal programme. During the above period, the removal programme was carried out in the society of Dutch cat breeders 'Felikat', the programme being made compulsory on all members of the society. The incidence of cats positive for FeLV decreased from over 11 per cent in 1974 to less than 2 per cent within four years. None of the cats tested in this society was found to be positive for FeLV in 1984 and 1985. Besides removal programmes, other methods of control, such as vaccination, were developed to prevent the spread of FeLV. The FeLV-immunostimulating complex vaccine (FeLV-ISCOM vaccine) a subunit vaccine in which FeLV-gp70 is presented in a particular manner, seems to be promising.