Lactobacilli are probiotics shown to have antitumor activities. In addition, they can regulate gene expressionthrough epigenetic mechanisms. In this study, we aimed to assess anti tumor activities of Lactobacillus acidophilusand Lactobacillus crispatus on the MDA-MB-231 breast cancer cell line. The effects of culture supernatantswere determined by MTT [3-(4,5-dimethylthiazol-2-y-2,5-diphenyltetrazolium bromide] assay. Changes inexpression of 5 cancer-testis antigens (CTAs), namely AKAP4, ODF4, PIWIL2, RHOXF2 and TSGA10 ,wereanalyzed by quantitative real time RT-PCR. The culture supernatants of the 2 lactobacilli inhibited MDA-MB-231cell proliferation. In addition, transcriptional activity of all mentioned CTAs except AKAP4 was significantlydecreased after 24 hour treatment with culture supernatants. This study shows that Lactobacillus acidophilus andLactobacillus crispatus have antiproliferative activity against MDA-MB-231 cells. In addition, these lactobacillicould decrease transcriptional activity of 4 CTAs. Previous studies have shown that expression of CTAs isepigenetically regulated, so it is possible that lactobacilli cause this expression downregulation through epigeneticmechanisms. As expression of CTAs in cancers is usually associated with higher grades and poor prognosis,downregulation of their expression by lactobacilli may have clinical implications.