We investigated the effect of ingesting Lactobacillus pentosus S-PT84 on the interferon-α (IFN-α) production from splenocytes and plasmacytoid dendritic cells by virus stimulation. IFN-α production by the Lactobacillus pentosus S-PT84 ingestion group was significantly greater under the virus-infected condition than that by the control group. Lactobacillus pentosus S-PT84 could enhance the production of IFN-α which is known as an important cytokine for preventing virus infection. It may therefore become a prophylactic tool against such virus infection.