Histidine appears to be an essential amino acid for the young infant. Its omission from the diets of young infants gives rise to a depression of weight gain and of nitrogen retention. It also resulted in a dermatitis clinically and pathologically similar to infantile eczema, except for the absence of pruritus and atrophic changes in the sebaceous glands. Under the conditions of this study, the histidine requirement was less than 35 mg/kg/day in all six infants tested with this intake. Five infants were given a trial of 22 mg/kg/day; in three this figure was satisfactory, but in the remaining two there was some evidence of inadequacy. An intake of 16.6 mg/kg/day appeared to be adequate for one infant.