Significant emphasis is being placed on combination chemotherapy of cancer using cytotoxic agents and naturally occurring chemopreventive agents, having different mechanisms of action with non-overlapping toxicity. In this regard, here we assessed whether a cancer preventive agent silibinin synergizes the therapeutic potential of doxorubicin (Dox), cisplatin or carboplatin, the chemotherapeutic drugs, in both estrogen-dependent and -independent human breast carcinoma, MCF-7 and MDA-MB468 cells, respectively. When tested alone, each of the four agents showed growth inhibition in both the cell lines in a dose- and a time-dependent manner. Based on their growth inhibitory effects, several combinations of silibinin (25-100 microM) with Dox (10-75 nM), cisplatin (0.2-2 microg/ml) or carboplatin (2-20 microg/ml) were next assessed for their synergistic, additive and/or antagonistic efficacy towards cell growth inhibition and apoptotic death. The strongest synergistic effects for cell growth inhibition [combination index (CI) 0.35 for MCF-7 and 0.45 for MDA-MB468 cells] were evident at a silibinin dose of 100 microM plus 25 nM Dox, in both the cell lines. Most of the CIs for other combinations of these three drugs with silibinin also suggested strong synergistic effects for cell growth inhibition in both MCF-7 and MDA-MB468 cells. In quantitative apoptosis studies, combination of silibinin with Dox resulted in much stronger apoptotic death compared to each agent alone in both cell lines. In case of silibinin combination with cisplatin, it showed no additional apoptotic effect in either cell line. Similarly, silibinin plus carboplatin combination showed stronger apoptotic effect only in MCF-7 cells. Together, these results suggest a possible synergism between silibinin and conventional cytotoxic agents for breast cancer treatment, and warrant further in vivo studies in pre-clinical breast cancer models.