Lactic acid bacteria (LAB) are of interest because of their potential to modulate immune responses. The effects of LAB range from regulation to stimulation of the immune system. It has been reported that LAB affect health via two main mechanisms: directly through physical interactions between LAB and cells of the immune system, and indirectly through the products of these bacteria. The studies presented in this thesis examine the direct and indirect effects of LAB on the immune system specifically on murine dendritic cells (DCs). Mouse DCs (in form of the DC2.4 cell line) were treated in vitro with a fraction of bovine milk fermented with Lactobacillus helveticus-2 (LH-2) or three synthetic peptides identified within the fermented milk fraction. Cell culture supernatants were analyzed for presence of tumor necrosis factor (TNF)-α and interleukin (IL)-6 to determine the effects of LAB on DC activation. The results of this study showed that the ability of the milk derived fraction and the synthetic peptides to induce DC activation and production of pro-inflammatory cytokines was limited, suggesting that these peptides may induce regulatory immune responses. A series of studies was performed in vitro to investigate the effects of six LAB species and strains, (LH-2), Lactobacillus acidophilus-5 (La-5), Lactobacillus acidophilus-115 (La-115), Lactobacillus acidophilus-116 (La-116), Lactobacillus acidophilus-14 (La-14), and Lactobacillus salivarius, on maturation and activation of DC2.4. Production of TNF-α, IL-6 and IL-10 by DCs was determined after treating cells with live LAB. The expression of DC maturation markers, CD80 and CD40, was also measured using flow cytometry after stimulation with LAB. In addition, the expression of toll-like receptors (TLRs) 2, 4 and 9 by DCs stimulated with LAB was measured. Our results revealed that LAB act differentially on pro-inflammatory and anti-inflammatory cytokine production and induction of co-stimulatory molecules by DCs. Specifically, L. salivarius was found to be the most effective LAB to induce pro-inflammatory cytokine production and expression of co-stimulatory molecules. Moreover, La-14, La-116 and La-5 induced moderate maturation and activation of DCs. On the other hand, LH-2 and La-115 are the least likely lactobacilli to induce DC response. In conclusion, various strains and species of LAB can differentially regulate DC activation and maturation, raising the possibility that these microbes can influence and steer immune responses of the host.