We report the development of a retroviral vector system in which two dominant oncogenes are expressed inducibly in human cells using the lac repressor/lac operator regulatable promoter system. First, the parent vector, pLoCRNLo, was constructed to contain a retroviral long terminal repeat (LTR) promoter that has been modified by incorporation of a lac operator sequence (lacO). This promoter, LTRo, was shown to mediate IPTG-inducible cat expression in rat cells expressing the lac repressor. The pLoCRNLo backbone was used to develop the retroviral vector LoTPRRNLo which expresses SV40 T antigen and H-ras val12 oncogenes as a dicistronic unit separated by a poliovirus internal ribosome entry sequence (PO-IRES). LoPRRNLo retrovirus was produced as a VSV-G protein pseudotype and used to infect primary human cells, resulting in the efficient formation of transformed cell lines. Subsequent introduction into the transformed cells of the lac repressor, expressed from a second retroviral vector, MSCV-In(S), resulted in IPTG-responsive oncogene expression and cell growth. This vector system is useful for introducing multiple genes under inducible control into mammalian cells.