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This is a review of previously published data from experiments designed to measure the chicks' requirement for an essential amino acid in diets containing surplus protein (generally in the range 220–300 g crude protein (CP)/kg diet). The evidence shows that, within this range, the requirement for a first limiting amino acid increases nearly in direct proportion to the CP content of the diet. To explain this, the following possibilities are considered: (1) that energy supply was limiting the response to a critical amino acid, (2) that the experiments were conducted in environments which limited heat disposal by the chicks, (3) that results from the trials have been misinterpreted because growth rate rather than protein deposition was used as a response measure, (4) that the availability of amino acids in the diets was lower than had been assumed, (5) that nutrients other than the amino acid under study were limiting …