We have studied the lac repressor (lacR) system in two breast cancer cell lines, MCF-7 and MDA-MB-231, in vitro and in vivo. Breast cancer cell lines were stably transfected with lacR and tested for inducibility by transient transfection with a lac operator/luciferase reporter plasmid. The level of expression of lacR did not appear to correlate with the basal or maximal activation of induction by isopropyl β-D-thiogalactoside (IPTG). Stable transfection with the same reporter gene resulted in up to 40-fold (MDA-MB-231) and 50-fold (MCF7) induction. In the absence of IPTG, a low level of basal reporter gene expression was seen in all clones. Detailed analysis showed that induction was rapid (maximal at 24 h), reversible (a return to basal expression by 24 h) and dose-dependent. To test if this system was also inducible in vivo, cells were grown as a xenograft tumor in nude mice. Mice were given IPTG (0.53 mmol) by intraperitoneal injection, and the tumors were biopsied at several time points following administration. IPTG caused a 10-fold increase in luciferase activity after 8 h, which persisted for 24 h. Thus, this system allows tightly controlled inducible in vivo and in vitro gene expression with low basal expression, and it may provide an important tool for the study of lethal genes in human breast cancer cells.