Background: Probiotic microorganisms are living normal flora of human body that have nutritional value and health benefits when administered in adequate amounts. The health benefits include prevention of bacterial diarrhea, skin eczema and recently understood, prevention and control of various cancers, as well. Different mechanisms such as stimulating the immune system, modifying the composition of gastrointestinal and genitourinary tract normal flora and prevention of the carcinogenic activity of fecal enzymes have been identified for their probiotic activity. Due to the high density of the normal flora in the gut and also preferentially sporadic nature of colorectal cancers, these cancers are among the main candidates of treatment trials with probiotics. In this study, direct effects of probiotic lactobacilli on colon cancer tumor cells were studied. Methods: Supernatant fluid and bacterial extracts were prepared and CaCo-2 cells were treated by these materials. Subsequently, the effects of the aforesaid elements were evaluated on cell proliferation, cell necrosis and cell apoptosis by MTT assay, LDH assay and caspase-3 activity. Results: The supernatants of lactobacilli decreased cell proliferation and increased cell apoptosis but they did not have any effect on cell necrosis. In contrast, when cancerous cells were treated by lactobacilli extract, it lead to cell necrosis in addition to reduction in cell proliferation and increase in cell apoptosis. Conclusion: The use of lactobacillus probiotics may reduce proliferation of tumor cells in the early stages of colorectal cancers.