1. Protein utilization was determined by assay with low-protein diets limiting in one of nine essential amino acids. The diets contained 50 g protein/kg and the amino acid score of the protein was 0.50.
2. Protein utilization was found to be related to the particular limiting essential amino acids present.
3. The usefulness of Bender's (1965) standard amino acid requirement for the estimation of amino acid utilization from a low-protein diet is discussed.