The bifidobacteria are among the microbial genera normally found in the gastrointestinal tract of pigs and other mammals. They are claimed to beneficially influence the health of the host by protecting against intestinal infections and have therefore been advocated and used as probiotics in man and animals. The purpose of this study was to investigate the bifidobacteria of the pig gut with a view to selecting strains suitable for probiosis. Bifidobacteria were isolated from the faeces of pigs by quantifying fermentation end-products and by assaying for the unique bifidobacterial enzyme fructose-6-phosphate phosphoketolase. Species identification was achieved by analysis of carbohydrate utilisation ability and isozyme mobility on polyacrylamide gels. A number of bifidobacteria of indeterminate species were found. Bifidobacterial isolates were further characterised to identify their resistance to heat and bile salts and sensitivity to oxygen. Certain indigestible carbohydrates have been proposed to selectively stimulate bifidobacteria in the hindgut when fed to animals. A range of these carbohydrates were screened for ability to support growth of the isolates in vitro using a microtitre plate assay. Bifidobacteria were screened for antagonistic activity against a number of intestinal pathogens and other bacteria likely to compete for the same ecological niche. A number of inhibitory strains were found. The inhibitory effect was due solely to the acidic end-products of bifidobacterial fermentation. A mixed flora fermenter simulation of the porcine ileum was used to study the effect of bifidobacterial and fructo-oligosaccharide supplementation on the survival of an enterotoxigenic Escherichia coli strain. Although the supplementation did not increase the rate of disappearance of the E. coli strain from the fermenter contents, an increase in acid production by the fermenter contents was noted. This may represent an advantageous consequence of bifidobacterial fermentation in vivo.