Overexpression of the anti-apoptotic proteins bcl-2 and bcl-xL is implicated in breast cancer development, tumor progression and drug resistance. Here we describe the use of the bcl-2/bcl-xL bispecific antisense oligonucleotide 4625 to sensitize breast carcinoma cells to anti-cancer drugs routinely used in breast cancer therapy. MCF7 cells were treated with oligonucleotide 4625, doxorubicin, paclitaxel or cyclophosphamide alone, or with combinations of oligonucleotide and the anti-cancer drugs. As measured in cell viability assays, treatment with the various combinations reduced the number of viable MCF7 cells more effectively than treatment with the single drugs alone. Treatment with a sequence control oligonucleotide did not affect cell viability. All combination treatments induced apoptosis as demonstrated by the appearance of massive nuclear condensation in a high proportion of the cells. To further characterize the interaction between 4625 and doxorubicin, paclitaxel or cyclophosphamide, the median-effect method was used. In MCF7 cells all combinations resulted in potent synergistic effects over a broad range of toxicity with combination indices ranging from 0.8 to 0.1. Similarly, strong synergistic interactions between oligonucleotide 4625 and the anti-cancer drugs were also observed in cultures of the breast carcinoma cell line MDA-MB-231. Our data suggest the use of 4625 as a potent adjuvant in breast cancer chemotherapy.