The ultraviolet absorption band of tyrosine was found to be remarkably lowered and shifted toward shorter wavelengths by treatment with cyanuric fluoride, (CNF)3, whereas the bands of other amino acids were either unaffected by the treatment or affected below 295 mμ, the peak position in the difference spectrum of the ionization of tyrosine. By use of this phenomenon, the reactivities of the tyrosine residues of lysozyme and insulin with cyanuric fluoride were examined, and the following facts were established. Two of the three tyrosine residues in the lysozyme molecule react with cyanuric fluoride and the remaining one is non-reactive. Of the four tyrosine residues in the insulin molecule, two are reactive while the other two are non-reactive. Upon addition of alkali to an insulin solution, one of the two non-reactive residues is transformed rapidly and the other slowly into the reactive type. Of these two bound residues of the non-reactive type one is in the A the other in the B chains of insulin. The numbers and the positions of the bound residues of the same proteins, identified by other properties, are discussed.