The anti-listerial effect of pediocin PA-1 and its producing strain, Pediococcus acidilactici UL5, was investigated in vivo using an ICR mouse model. The effect of intra-gastric administration of a single dose of P. acidilactici UL5 (4 x 10(10) CFU/animal) on the propagation of Listeria monocytogenes LSD348 in intestine, liver and spleen was negligible. P. acidilactici UL5 did not appear competitive with the mouse intestinal flora and was not detectable in fecal samples collected two days after administration. However, double-agar-layer activity assay showed the ability of P. acidilactici UL5 colonies recovered from fecal samples one day after administration to produce pediocin PA-1 and inhibit L. monocytogenes. Moreover, repeated doses (250 microg/day for three consecutive days) of purified pediocin PA-1 provided up to 2-log reductions in fecal listerial counts compared to the infected control group and slowed pathogen translocation into the liver and spleen, leading to the disappearance of L. monocytogenes infection in these two organs within six days. Neither P. acidilactici UL5 nor ingested purified pediocin PA-1 had any negative effect on feed intake or body weight development. Pediocin PA-1 did not affect the composition of the mouse intestinal flora, suggesting a potential advantage over other inhibitory agents as a prophylactic measure against L. monocytogenes.