The effect of diet on intestinal ecology was studied in germ-free mice that were inoculated orogastrically with predominant intestinal flora components isolated from the feces of breast-fed human infants. The flora components colonized the intestines of mice and persisted at fixed population levels. Groups of flora- associated mice were fed either human milk, bovine milk, whey-dominant formula, or formula modifications exclusively for 2 weeks, and then examined for changes in small intestinal and cecal flora composition, cecal pH, and resistance to intestinal colonization with Salmonella typhimurium. Dietary variations influenced the composition of the flora to a moderate degree but the differences were generally not statistically significant. However, the addition of bovine lactoferrin to the whey-dominant formula resulted in significantly greater counts of Bifidobacterium, Bacteroides, Enterococcus and total aerobes in the small intestine when compared with mice fed unsupplemented formula. Bifidobacterium was present in large numbers in both the ceca and small intestines of mice fed the lactoferrin-supplemented formula. Despite similarities in intestinal flora patterns among mice fed the various diets, human milk consumption resulted in a lower pH of cecal contents and a greater resistance to colonization by Salmonella typhimurium after orogastric challenge than the consumption of the other diets.