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1. Three experiments were designed to determine the response of broiler chickens to dietary isoleucine, and to quantify the antagonistic effects of excess leucine and valine on this response.

2. A dilution technique was used to measure the responses in growth rate and food intake to a range of diets differing in their isoleucine concentrations. A summit diet was formulated to contain isoleucine at 1.14 times the requirement and with leucine (1.76 times the requirement) and valine (1.87 times the requirement) at the minimum possible concentrations, given the ingredients available. A dilution mixture, devoid of protein, was formulated to correspond in all respects, other than in amino acid content, to the summit diet. These two basal diets were blended in different proportions to give a range of diets of decreasing isoleucine and protein content.

3. In experiment 1 the response was measured to isoleucine with leucine and …