Hand rearing infants has always carried a greater mortality rate than breast feeding despite the development of safe preserving techniques for milk: stool bacterial culture revealed two distinctive florae, fermentatitive, with a predominance of lactobacilli in breast fed infants and putrefactive, with a predominance of coliform bacteria in artificially fed infants. This difference was suggested to be the basis for the difference in the incidence of gastroenteritis. An artificial milk inducing a breast milk stool flora would, thus, reduce the incidence of gastroenteritis in artificially fed infants. The buffer capacity of milk was claimed to determine stool buffer capacity, which then encouraged growth of specific bacterial populations. However, an artificial milk with identical buffer capacity to breast milk returned disparate bacterial results. The hypothesis examined in this study was that buffer capacity of infant milk modifies stool flora. Breast milk and three artificial milks were studied. The artificial milks differed either in buffer capacity or in composition of major dietary components. Biochemical analyses were performed on milk, stool and stool water obtained by ultracentrifugation. Buffer capacity in milk was related to protein content. No biochemical differences were detected in stool water from each group, however whole stool buffer capacity was considerably greater in the group fed high buffer capacity milk than in the other three groups. No difference was detected in total bacterial numbers cultured, the types of organisms, nor individual species numbers. Thus the presently available highly modified artificial milks may determine stool buffer capacity, but do not induce a gut flora distinct from breast feeding. Thus the continuing increased incidence of gastroenteritis in artificially fed infants can no longer be related to differences.