Bovine leukaemia virus (BLV) propagated in a cell clone of foetal lamb kidney cells was transmitted by cell contact to bovine and ovine embryo cells derived from different organs. The transmission of the BLV genome was effectively achieved by cocultivation of mitomycin C-killed, virus-productive cells of the cell clone with embryo cells or by cell fusion. Infection of the same embryo cells by the virus was less effective. The transmission of BLV genome from nonproducer cells by the same procedure only exceptionally succeeded. The donor cells contained 3 integrated BLV proviruses. In the cells with transmitted BLV genome one provirus was found only. The majority of cells contained both unintegrated and integrated BLV provirus. In the cells containing the transmitted BLV, the virus genome was expressed to its protein products, some cells produced virus particles and reverse transcriptase activity into the medium. The implications of these findings are discussed with regard to biological and molecular properties of the retrovirus family to which the BLV belongs.