It seems probable that several factors must be considered jointly in explaining the falsely high body content of protein calculated on the basis of nitrogen balance studies. Of these, the most important are probably cumulative errors in balance techniques, nitrogen loss in sweat and desquamated epithelium, and differences in retention of nitrogen during balance studies and in intervals between balance studies. Confirmation or refutation of the conclusion based on metabolic balance studies—that feeding of high-protein diets results in accumulation of relatively large amounts of protein in the body—must certainly come from assessments of body composition of normal infants made with techniques other than metabolic balance studies. Similar errors are inherent in calculating body content of calcium, phosphorus, sodium, and other minerals from results of metabolic balance studies. However, in these instances the systematic procedural error will be relatively greater because a smaller percentage of the intake is retained. With some substances, e.g., sodium and potassium, failure to measure losses from skin will introduce a much greater error than is the case with nitrogen. The unsuitability of metabolic balance studies as a means for estimation of body content of a substance indicates that the absolute values obtained with the methods are not precise. The methods are nevertheless useful when only relative results are sought, as in the nutritional evaluation of two foods fed to comparable groups of infants at similar intakes of nitrogen or in comparison of performance of normal subjects with subjects having certain metabolic abnormalities. The usefulness of balance technique is also obvious in the study of patients in whom large retention or losses are to be anticipated.