Consumption of a Lactobacillus helveticus SBT2171 (LH2171)-containing cheese has been reported to exhibit immunoregulatory actions, including an increase in regulatory T cell population and reduction in proinflammatory cytokine production in mice. We examined the in vitro effects of LH2171 cells per se on immune cell function, specifically proliferation and cytokine production, which are primary reactions of the immune response. Immune cell fractions were prepared by mechanical disruption of mesenteric lymph nodes (MLN), Peyer's patches (PP), and spleens (SP) of mice. The cell fractions were dispensed into a culture plate and stimulated with anti-CD3/CD28 antibody beads in place of antigen-presenting cells or lipopolysaccharide (LPS) in the presence or absence of heat-treated LH2171 cells and other bacterial strains for comparison. After incubation, proliferation, cytokine production, and cell viability of the immune cells were determined. The LH2171 significantly inhibited the proliferation of MLN immune cells stimulated with anti-CD3/CD28 compared with other bacterial strains. The antiproliferative potency of LH2171 was effective not only on MLN but also on PP and SP stimulated with anti-CD3/CD28 or LPS. The LH2171 also decreased LPS-stimulated IL-6 production from MLN, PP, and SP, and IL-1β production from SP, but LH2171 did not affect the viability of immune cells. The LH2171 inhibited immune cell proliferation and proinflammatory cytokine (IL-6 and IL-1β) production. The inhibitory actions were not due to cytotoxicity to immune cells, suggesting that LH2171 is a dairy Lactobacillus strain with beneficial immunoregulatory properties.