Some complex amino acids[long dash]-phenyl-alanine, tyro-aine, tryptophane[long dash]accelerated differentiation in amphibian larvae in a fashion qualitatively similar to, but quantitatively less than that induced by diiodo-tyrosine or thyroxine. The degree of acceleration increased with the complexity of the acids. These "differentiation" acids supplemented the action of diiodo-tyrosine. Some less complex amino acids influenced growth but not differentiation; others, still less complex, were sufficient only for maintenance.