Lactic acid bacteria (LAB) form a major component of gut microbiota and are often used as probiotics for fermented foods, such as yoghurt. In this study, we aimed to evaluate immunomodulatory activity of LAB, especially that of Lactobacillus bulgaricus ME-552 (ME552) and Streptococcus thermophilus ME-553 (ME553). In vivo/in vitro assay was performed in order to investigate their effects on T cell function. After oral administration of ME553 to C57BL/6 mice, the amount of both interferon γ (IFN-γ) and interleukin 17 (IL-17) produced by cluster of differentiation (CD) 4⁺ T cells from Peyer’s patches (PPs) were significantly enhanced. On the other hand, ME552 only up-regulated the production of IL-17 from PP cells. The extent of induction for IFN-γ production differed between ME552 and ME553. These results suggest that LAB modulate T cell effector functions and mucosal immunity.