Human T-lymphotropic viruses (HTLV-I and -II) and bovine leukaemia virus (BLV) express transactivator proteins able to increase long terminal repeat (LTR) directed viral expression. These transacting factors are though to be involved in the induction of leukaemia by these viruses. Transfection of BLV transactivator p34tax together with Ha-ras immortalizes and transforms rat embryo fibroblasts, in vitro. The transformed cell induce tumours in nude mice. These data emphasize the causal role exerted by p34tax in in vivo tumorigenesis.