Abstract In a previous report, Klain et al. (1959), it was shown that a mixture of crystalline amino acids simulating casein, supported excellent chick growth when the concentration of arginine was increased to 2.06 percent of the diet. Of equal significance was the improved gain-feed ratio that resulted when arginine was adequate. In all prior experiments with crystalline amino acid diets the gain per unit of feed consumed has been substantially inferior to that noted on the simulated casein. Since a major difference between the amino acid mixtures was the presence of certain non-essential amino acids in the simulated casein mixture it seemed desirable to ascertain whether (1) the amino acid diet would perform equally as well when the “non-essential” amino acids (aspartic acid, proline, alanine and serine) were omitted from the mixture and (2) whether there is an interaction between these amino acids and arginine. The amino acid mixture simulating …