Abstract Estimated human protein requirements have been substantially lowered by FAO/WHO expert committees over the past two decades. The estimates and methods of calculation are considered in the light of the kinetics of response to protein intake, body protein turnover, amino acid flows in the body, and the concept of nitrogen (N) steady state. Whereas traditional methods of estimation have assumed an essentially linear (first order) response of N retention to absorbed N, animal studies show that response to graded protein intakes obeys saturation kinetics. Corrections for protein quality have also assumed a linear relation between response and supply of limiting amino acid, while animal experiments indicate that this response likewise follows saturation kinetics. Evidence is lacking that the present minimum protein standards for humans can support acceptable internal nitrogen steady states at any age above infancy or foster normal growth in the child. New research approaches to determination of protein requirements are suggested, including study of the kinetics of human response to graded protein intakes and graded variations of quality; development of indicators of nitrogen steady state and correlation with clinical status; and determination of optimum protein-energy ratios by age and sex.