Enteric pathogens such as Escherichia coli 0157:H7 and Listeria monocytogenes have been identified as an important cause of human intestinal disease. Cattle and other ruminants appear to be the main reservoir of these food-borne pathogens. Reducing the carriage and shedding of these food-borne pathogens from their animal host would be of significance both for public health and economically. The objective of the present study was to investigate the effect of S.· cerevisiae on growth and/or survival of E coli 0157:H7 and L. monocytogenes in the rumen, using a combination of in vitro and in vivo experiments. Batch culture incubations showed that not all strains of S. cerevisiae could prevent the proliferation of E. coli 0157:H7 strain 12900R and L. monocytogenes in rumen fluid. This was confirmed in continuous culture (using the Rumen Simulation Technique: Rusitec) which demonstrated that somibut not all strains of yeast, could prevent the proliferation of E. coli 0157:H7 and L. monocytogenes and this anti-pathogenic effect seemed at least in part related to ability of the different strains of yeast to stimulate bacterial numbers in rumen. However, the distribution of the main bacterial species as indicated by 16S rDNA PCR-DGGE did not respond to yeast addition. Inoculation of pathogens into the rumen of multicannulated sheep in controlled feeding trials showed that 5 g d-1 of yeast tended to prevent the numbers of Listeria innocua from the rumen and this was associated with an increased total bacteria population. However, again the yeast did not have an effect on bacterial diversity assessed by 168 rONA PCR-DGGE. Furthermore, despite the decrease in pathogens leaving the rumen; yeast had no effect on shedding in the faeces. In a cattle trial with animals exposed to constant natural challenge of 20 g d-1 of yeast decreased the flow of Listeria leaving rumen and seemed to have an effect on pathogen shedding in faeces. In conclusion some but not all strains of S. cerevisiae reduced pathogen survival in the rumen; however, this was not always reflected in a decrease in faecal shedding.