Background: In the field of human breast cancer, the most reports referred to the influence of endogenous estrogens and/or exposure to the estrogen mimetic agents, as risk factors. Zearalenone (ZEN) as a mycotoxin and its derivative α- zearalenol (α-ZOL) are known non-steroid estrogenic compounds with potential endocrine disrupting properties. The present study was designed to investigate the effects of ZEN and α-ZOL on MCF-7 cell line of human breast cancer and MDAMB- 468 estrogen receptor knock-out gene cell line of human breast cancer. Methods & Materials: Cell lines were treated by low and high doses of ZEN and α-ZOL (0, 1, 30, 62, 125, 250 and 500 ng/ml and 1, 2, 4, 8, 15, 30, 62 and 125 μg/ml) for 24 and 48 hours. Then, MTT colorimetric assay was used to evaluate the cytotoxicity effect of ZEN and α-ZOL. Furthermore, morphological changes of treated and untreated cell lines were studied under an inverted microscope. Results: The results obtained from the present study were demonstrated that both ZEN and α-ZOL increase the cell viability of MCF-7 especially at low doses (1-500 ng/ml) and at a high dose of 125μg/ml after 24 and 48 h; however, this effect for α-ZOL was somewhat greater than that for ZEN. On the other hand, these estrogenic compounds didn´t had any effect on the cell viability of MDA-MB-468. No morphological change were observed in treated cells. Conclusion: In conclusion, these results indicate shows that ZEN and α-ZOL enhance the cell viability through the estrogen receptor and therefore exposure to this mycotoxin may increase the risk likelihood of breast cancer risk.

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