Interleukin 1β-converting enzyme (ICE) is a member of a growing family of cysteine proteases shown to be a crucial component in the activation of a genetic program that leads to autonomous cell death in mammalian cells. In this study, a murine ICE-lacZ fusion gene was introduced into a novel retroviral vector designed to achieve regulated ectopic expression of a foreign gene in mammalian cells. By delivering the ICE-lacZ gene within a retroviral vector and under the control of a tetracycline-regulated promoter, we were able to utilize the intrinsic cell death program of ICE as a means for tumoricidal therapy in a rat brain tumor model. Both in culture and in vivo suppression of ICE-lacZ expression was extremely tight in the presence of tetracycline, as determined by the lack of X-galactosidase-positive tumor cells and by cell viability. When tetracycline was withdrawn, ICE-lacZ gene expression was rapidly turned on and apoptosis-mediated cell death occurred in essentially all tumor cells.