After a short historical introduction and a description of some of the properties of the bovine leucosis (leukemia) virus (BLV) the natural pathways of transmission, for which most likely intact infected cells are a prerequisite, are described and the role of possible vectors mentioned. The iatrogenic transmission by needles, dehorning and perhaps even rectal examinations is discussed. Semen, embryo transfer, saliva, urine, faeces and breath do obviously not play any role in contrast to nasal secretions containing viable cells. It is pointed out that it is not inevitable to eliminate all seropositive cattle from the premises during an eradication programme provided some additional tests are carried out: determination of the white blood picture, the p 24 status and perhaps even proof of antigen production in short-term buffy coat cultures. Offspring of seropositive cattle requires some special attention and careful testing at certain intervals. The report according to which BVD/MD virus-infected cattle are superinfected with BLV leading to rather difficult situations is cited.