Lymphocytes serve an important function in mediating specific immune responses. When the body is stimulated by internal or external antigens, activated lymphocytes proliferate to clear pathogens by secreting antibodies or cytokines. Some bioactive peptides were isolated from fermented milk in previous studies. One of the peptides, Gln-Glu-Pro-Val-Leu (QEPVL), was synthesized and used in this experiment. Results show that QEPVL can significantly activate lymphocytes both in vitro and in vivo. QEPVL can also increase the lymphocyte proliferation rate and cyclic AMP levels. This positive regulation had a dose-effect relationship within certain concentration ranges. QEPVL can also inhibit LPS-induced inflammation by regulating nitric oxide release and the production of the cytokines IL-4, IL-10, IFN-γ, and TNF-α in vivo. Digesting QEPVL in artificial gastrointestinal juice yields the digestion product Gln-Glu-Pro-Val (QEPV), which exhibits bioactivities similar to those of QEPVL in vitro. Overall, QEPVL has significant immunomodulating effects on lymphocytes and contributes to inflammation treatment through the oral route as a functional food ingredient.