A scrutiny of the extensive data obtained from the metabolic study of infants in a state of chronic malnutrition reveals the fact that the fundamental condition is a lack of utilization by the organism of nourishment. Despite the high calorific value of the diet, there is no increase in body weight. The cause of this imperfect utilization has not yet been established. It seemed to us that there was one possible factor, the study of which has been particularly neglected, namely: the correlation of the influence of intestinal bacterial flora and the utilization of the ingested foodstuffs. An investigation of this question was the original object of the work undertaken in this study. It was planned to alter the diet of the infants in a manner that in different periods it would contain a maximum of one of the three principal food constituents — protein, fat or carbohydrate — and