Effect of L-phenylalanine on both anaerobic glycolysis and growth of tumor cells was examined. L-Phenylalanine in 20mM concentration inhibited glycolysis of Ehrlich ascites tumor cells by 50%. Growth of the ascites cells was suppressed by feeding of the host mice with a diet containing 7% L-phenylalanine. This effect seemed highly specific to phenylalanine since no suppressive effect was observed by administration of other ammo acids, including L-alanine, L-glutamic acid, glycine, L-histidine, L-isoleucine, L-leucine, L-lysine, L-methionine, L-proline, L-threonine, L-tryptophan, L-tyrosine, and L-valine. Phenylalanine did not show any suppressive effect on growth of the tumor cells which had been mainitained for 11 generations in mice fed the phenylalanine diet. L-Phenylalanine could not suppress the growth of Walker tumor or the rat ascites hepatoma AH-130 cells.