Gastrointestinal survival of the bacteriocin-producing strain, Lactococcus lactis DPC6520, was evaluated systematically in vitro and in vivo with a view to using this strain to deliver biologically active lacticin 3147, a broad-spectrum bacteriocin, to the gut. The activity of the lacticin 3147 producer was also evaluated against two clinically relevant pathogens: Clostridium difficile and Listeria monocytogenes. When suspended in an appropriate matrix, the lactococcal strain is capable of surviving simulated gastrointestinal juices similar to the porcine probiotic, Lactobacillus salivarius DPC6005. Upon administration of L. lactis DPC6520 to pigs (n=4), excretion rates of ∼10(2) -10(5) CFU g(-1) faeces were observed by day 5. Although passage through the gastrointestinal tract (GIT) did not affect lacticin 3147 production by L. lactis DPC6520 isolates, activity was undetectable in faecal samples by an agar well diffusion assay. Furthermore, L. lactis DPC6520 had no inhibitory effect on C. difficile or other bacterial populations in a human distal colon model, while lactococcal counts declined 10,000-fold over 24 h. The lacticin 3147 producer failed to prevent L. monocytogenes infection in a mouse model, even though a mean L. lactis DPC6520 count of 4.7 × 10(4) CFU g(-1) faeces was obtained over the 5-day administration period. These data demonstrate that L. lactis DPC6520 is capable of surviving transit through the GIT, and yet lacks antimicrobial efficacy in the models of infection used.