In previous study of normal infants fed pooled, pasteurized human milk ad libitum with additional vitamins (A, C, and D) but no other supplementary foods,1the mean daily intake of protein between 4½ and 6 months of age was found to be 1.7 gm. per kilogram (standard deviation ±0.3). It has long been customary to supply considerably greater intakes of protein to infants receiving formulae, on the reasonable assumption that too little protein was likely to be more detrimental to the infant than an excess; there is as yet no answer to the question of whether protein supplied from other sources may enable infants to thrive as well as does human milk. The present study was designed to test the effect of supplying a significant proportion of the infant's intake of protein from another source while keeping the total intake of protein similar to that of infants reared on