Allogeneic bone marrow transplantation and donor lymphocyte infusion are powerful treatments for chemotherapy-resistant leukemia. Tumor eradication is attributed to a graft-versus-leukemia reaction by the donor-derived cytotoxic T lymphocytes (CTLs), but the same cell population may cause severe graft-versus-host disease. One strategy to suppress harmful CTL activity is to incorporate a suicide gene into the donor lymphocytes prior to infusion, and to destroy these cells when they aggressively attack nonmalignant host tissues. In this study, we investigated the feasibility of using a Fas-estrogen receptor fusion protein (MfasER) to control T cell-mediated cytotoxicity, based on our previous finding that the chimera transmits a Fas-mediated death signal through activation by estrogen binding. A murine CTL line CTLL-2 was transfected with a vector encoding MfasER, and the growth, viability and cytotoxic activity of the transfected cells (CTLL/MfasER) were analyzed. The expression of apoptosis-related proteins such as Fas ligand and perforin was also investigated. In the absence of estrogen, CTLL/MfasER showed similar growth to parental CTLL-2, and the killing activity was preserved. Addition of 10−7M estrogen induced a rapid apoptosis of CTLL/MfasER, and the cytotoxicity was severely impaired. A decrease of Fas ligand and perforin in the estrogen-treated CTLL/MfasER was seen in an immunoblot analysis. These functional and biochemical analyses showed that the estrogen-inducible apoptosis in MfasER-expressing CTLs rapidly terminated their target cell killing. The feasibility of using the MfasER-estrogen system to control graft-versus-host disease was demonstrated.