Lactic acid bacteria which are used as probiotics have ability to modulate immune responses and modify immune mechanisms. It has also been indicated that some strains of this family can affect the immune responses against solid tumors. In the present work, we proposed to study the effects of oral administration of L.cacesi ssp casei on the NK cells cytotoxicity and also production of cytokines in spleen cells culture of BALB/c mice bearing invasive ductal carcinoma. 30 female In-bred BALB/c mice, were used and divided in two groups of test and control each containing 15 mice. Every day from 2 weeks before tumor transplantation 0.5 ml of PBS containing 2.7×108 CFU/ml of L.casei spp casei was orally administered to the test mice and it was followed 3 weeks after transplantation as well with 3 days interval between each week. Control mice received an equal volume of PBS in a same manner. Results showed that oral administration of L. casei significantly increased the production of IL-12 and IFN-γ (P<0.05) and increased the natural killer cells (NK) cytotoxicity in spleen cells culture of test mice (P<0.05). It has also been demonstrated that the growth rate of tumor in the test mice was decreased and their survival was significantly prolonged in comparison to the controls. Our findings suggest that daily intake of L.casei can improve immune responses in mice bearing invasive ductal carcinoma, but further studies are needed to investigate the other involving mechanisms in this case.