We applied the bacterial lactose and tetracycline repressor-operator systems to an interleukin S-dependent T-cell line, Kit 225, to examine the effects of the human T-cell leukemia virus type I oncogene product, Tax, on the cell cycle. The LacSwitch and Tet-Off inducible systems individually exhibited low expression of Tax upon induction in growing Kit 225 cells. In contrast, combination of the LacSwitch system with the Tet-Off system produced a high Tax expression level in growing Kit 225 cells; however when arrested at the G0/G1 phase of the cell cycle, Kit 225 cells expressed very low levels of Tax, associated with little or no cell cycle progression. Infection with the Tax recombinant adenovirus induced high expression of Tax and progression of the cell cycle. Our results indicate that the combined LacSwitch and Tet-Off systems may require cell growth for gene expression.