Rabbits and rats were inoculated with material derived from FLK cells producing permanently bovine leukaemia virus (BLV). The viral presence in the inoculum was proved by transmission electron microscopy, immunofluorescence, immunogold labelling demonstrating viral Tax protein, and PCR analysis. About 30 % of the infected animals sustained BLV seropositivity during the experiment, and demonstrated symptoms of lympholeukaemia - clinical manifestation of an immunosuppressive condition, increased number of lymphocytes and lymphoblasts, and preneoplastic lymphoid cell accumulations in the liver, lungs, kidneys, and lymph nodes. BLV DNA, detected by PCR in diseased animals, indicates the role of BLV as an aetiological factor of lympholeukaemia, developed in these animals after BLV infection. The alterations in rats were more pronounced than those in rabbits. The results prove that these two species of laboratory animals, especially rats, are suitable models for the in vivo studies of leukaemogenesis caused by BLV/HTLV infections.