The protein requirement and the efficiency of protein used were studied in young and old adult human subjects. Protein intake levels (N X 6.25) of 0.4, 0.8, and 1.6 g/kg body weight per day from a combination wheat-soy-milk mixture were fed. Caloric intake was held constant at 40 kcal/kg body weight per day throughout the 11-day study of each dietary period. No significant differences were observed in their protein requirement, efficiency of protein use or the ability to adapt to changes of protein intake levels. Protein digestibility was not impaired in the aged. It is concluded that the protein requirement and the efficiency of protein use are not affected by the aging process.