Phytochemicals have provided an abundant source of novel therapeutics for the treatment of human cancers. We have previously described a novel plant toxin, persin, derived from avocado leaves, which has unique in vivo actions in the mammary epithelium and Bim-dependent, cytotoxic effects in human breast cancer cells in vitro. Compounds structurally similar to persin, such as the polyunsaturated fatty acid, conjugated linoleic acid, can attenuate steroid hormone receptor signaling and modulate the response of breast cancer cells to antiestrogens. Here, we provide evidence that persin may have similar effects by showing its potent proapoptotic synergy with the antiestrogen 4-hydroxytamoxifen. However, although persin transcriptionally down-regulates estrogen receptor (ER) expression, unlike conjugated linoleic acid, it also shows efficacy in ER-negative breast cancer cells, both alone and in combination with 4-hydroxytamoxifen, whereas normal breast epithelial cells are unaffected, suggesting it may act via a distinct, ER-independent mechanism. These proapoptotic synergistic interactions are associated with increased de novo ceramide synthesis and are dependent on expression of the proapoptotic protein Bim. These data show that persin should be further investigated as a potential novel cancer therapeutic agent because it significantly enhances the sensitivity of breast cancer cells to the cytotoxic effects of tamoxifen, regardless of their ER status, while displaying apparent specificity for the malignant phenotype.