Mixture of protein foods given to chickens or rats frequently result in better growth than would be expected from the performance obtained with each component of the mixture given on its own. Mixing often results in the provision of a better amino acid balance in the diet and the improved growth can be attributed to the minimising of deficiencies of particular essential amino acids in the diet. This explanation is not always possible however. Some mixtures which give better growth have lower levels of some important amino acids than the better component of the mixture, and in such cases it is suggested that the improvements are due to the achievement of better overall amino acid balance. In particular it seems clear that amino acid excesses may have a deleterious effect, and diet balancing should be designed not merely to minimise deficiencies of essential amino acids, but also to cut down excesses.