Nutritive value of cereal protein is limited not only by deficiency of some essential amino acids like lysine but also by a number of other agents negatively influencing protein digestibility and its absorption. Evidence may be the frequently lower biological value of cereal protein that would be expected from its amino acid composition. Other agents limiting protein quality include: - the existence in grain of cereal of poorly digested pericarpium making the access of proteolytic enzymes to cell protein difficult; - the presence of antinutritive factors, first of all non-starch polysaccharides (pentosans, pectins, ß-glucans) building with some proteins in the small intestine indigestible complexes that lead to excessive development of microflora in the large intestine; - the existence in some kind of grains of natural contaminations, e.g. ergot, substantially limiting the use of these cereals in animal feeding.