Use of a diet containing mixtures of amino acids has shown the requirements of the chick of best growth to be: 1(+)-isoleucine, approximately 0.5% of the diet; l(-)-leucine, 1.5%; and l(+)-valine, 0.7%. d(-)-Isoleucine and d(-)-valine are not utilized for growth, but dl-leucine appears to be as effective as l(-)-leucine in promoting growth.