This investigation was undertaken to study the role of bifidobacteria in colonic nitrogen metabolism in CF1mice. Oral administration ofBifidobacterium pseudolongumisolated from the fecal material of CF1mice and the bifidogenic factor Neosugar significantly reduced plasma ammonia levels in animals fed with either a normal diet or a 50% protein diet. Changes in hepatic arginase activity and fecal excretion of ammonia and urea were not significant. Plasma ammonia concentration was inversely correlated with colonic bifidobacterial count. Cecal, colonic and fecal pH were significantly lower in animals fed bifidobacteria. Low intestinal pH has previously been shown to suppress the rate of ammonia production, trap ammonia in the lumen for bacterial assimilation, and enhance fecal elimination of ammonia in the form of bacterial proteins. The results of this study indicated that consumption of a host specific strain of bifidobacteria along with a growth factor can effectively alter the intestinal environment and induce favourable conditions on host metabolism such as in the management of elevated plasma ammonia levels.