Summary.None of the 13 most common amino acids show any activating effect on the reticulocyte ripening principle. Only tyrosine has this effect. Investigations on some tyrosine derivatives show that the phenol group is essential to the effect of tyrosine. The phenol group must be in para position to the side chain. The amino group can be replaced by an oxo‐, an oxy‐. or a methyl‐amino group. Minor changes in the molecule not affecting the OH group give only some reduction in the act