Experiments were designed with groups of broiler chickens to test whether the requirement for dietary lysine was related to the CP content of diets that had either a surplus or the recommended requirement of critical amino acids. Diets of equal energy were formulated using a summit and basal diet to obtain a range of CP contents from 14 to 26% (Experiment 1) with surplus critical amino acids or from 12 to 25% (Experiment 2) with the "ideal" amino acid balance. In both experiments at each CP level, there was an optimum lysine content for weight gain and feed conversion ratio (FCR). Diets with ideal amino acid balances gave less of a response as measured by growth rate and FCR than those with surpluses. Combining all data gave a significant improvement in both weight gain and FCR with increasing intake of dietary lysine. It would appear that because performance was highest on diets with the highest crude protein, there may be a requirement for CP per se. Alternatively, because all diets contained substantial amounts of synthetic amino acids, broilers may have a requirement for amino acid peptides that is not met by free amino acids.