Oligonol is a novel catechin-rich biotechnology product. The role of oligonol in modulating intracellular signaling mechanisms was investigated with the view of demonstrating its potential chemopreventive effect and the ability to inhibit cell proliferation using the estrogen-responsive MCF-7 and the estrogen-unresponsive MDA-MB-231 human breast cancer cell lines. Cell survival assay indicated that Oligonol was cytotoxic to both cells. Oligonol triggered apoptosis as revealed by the morphological features typical of nucleus staining and the accumulation of sub-G1 peak. Treatment with 25 microg/ml Oligonol resulted in an activation of caspase-7 and up-regulation of Bad on MCF-7 cells, while the Oligonol (20 microg/ml) induced up-regulation of Bcl-2 protein in a time-response manner on MDA-MB-231 cells. ERK1/2 in both cells were inactivated after Oligonol treatment in a time-dependent manner, and also inactivated upstream MEK1/2. Oligonol triggers apoptosis in MCF-7 and MDA-MB-231 cells through the modulation of pro-apoptotic Bcl-2 family proteins and MEK/ERK signaling pathway.