A discussion of the need for knowledge of amino acid requirements of infancy, as well as the methods for evaluating them, has been included in our previous publications on the threonine,1 phenylalanine,2 lysine,3 and valine4 requirements of infants. The present report is concerned with the estimation of the leucine requirements of the infant. The technique was the same as that employed in all our recent studies: the use of a synthetic diet, the nitrogen moiety of which was composed of a mixture of 18 L-amino acids in the proportion present in human milk. After a control period, the leucine was completely withdrawn from the diet and then reintroduced in stepwise fashion to determine the minimal quantity which permitted normal weight gain and nitrogen retention as well as the appearance of health. The nitrogen intake was kept constant by the substitution of glycine for the leucine