The nutritive function of proteins for monogastric animals may be simply to supply the body with amino acids. However, the complexities of digestion, absorption and metabolism imply that amino acid composition, determined chemically after hydrolysis, can only be a very rough guide (usually setting upper limits) to the nutritional value of proteins.Chemical reactions which may affect nutritional values of proteins are discussed at length. Chemical tests, for determining the extent to which these reactions have occurred, are increasing, both in number and usefulness. However, to derive practical benefits from chemical tests, frequent paxallel nutritional assessments must be conducted using animals.