We have previously reported that heat-killed Lactobacillus plantarum L-137 (HK-LP) stimulates macrophage/dendritic cells to produce T helper (Th) 1-related cytokines in vitro and in vivo in mice. We here examined the effect of oral administration of HK-LP on protection against influenza virus infection in mice. C57BL/6 mice were orally given HK-LP from day -7 to 7 and intranasally infected with influenza virus A/FM/1/47 (H1N1, a mouse-adapted strain) at 100 pfu on day 0. The survival time was significantly prolonged in mice treated with HK-LP than that in mice treated with PBS as controls. The viral titers in the lung were significantly lower in mice treated with HK-LP than controls at the early stage after influenza virus infection. An appreciable level of interferon (IFN)-beta was detected in the serum of mice treated with HK-LP, while no IFN-beta was detected in controls after influenza infection. Our results suggest that HK-LP, a potent IFN-beta inducer, is useful for prevention against influenza infection.