Resistance to apoptosis plays an important role in malignancies that are refractory to chemotherapy treatment. Recently we have shown that the expression of bax-alpha, a death-promoting member of the bcl-2 family, is down-regulated in breast cancer and have provided evidence that low bax expression might contribute to the pathogenesis of breast cancer. In this study we were able to demonstrate the role of this gene in chemotherapy-induced apoptosis. We transfected bax-alpha into the breast-cancer cell lines R30C and MCF-7 under the control of an inducible tetracycline-dependent expression system. Induction of bax-alpha expression did not affect viability by itself but strongly increased chemosensitivity to epirubicin. We were able to demonstrate that this sensitization is due to apoptosis. These data might explain the recently published observation that reduced expression of bax is associated with poor response rates to chemotherapy in breast cancer.