Tyrosine transamination has been investigatedin vitro with a preparation of rat liver tyrosine aminotransferase in the presence of several structural derivatives of the substrate, including the phosphonic analogue. The transamination by tyrosine aminotransferase (TAT) needs the presence in the substrate molecule of free amino and carboxylic groups, a three-carbon aliphatic chain, a para-phenolic hydroxylic function and aL-configuration. Some tyrosine analogues can markedly disturb the Tyr-TAT association: the chief structural modifications are (i) the removal of the free amine function in a compound still possessing a para-hydroxylic and a carboxylic group, (ii) the change of the carboxylic function by another acidic group, especially a phosphonic one, (iii) a disubstitution in positions 3 and 5. In every situation, the presence of a parahydroxylic group is compulsory to observe an inhibitory effect.