Tyrosine isomers and phenylalanine were separated by liąuid chromatography (HPLC). Since o- and m -tyrosine are formed as products of radiation induced conversion of phenylalanine in an aąueous Solutions, it was proposed to measure the level of these products as an indicator of irradiation of food products, particularly of those rich in proteins. The concentration of tyrosine isomers was found to increase with the increase of the radiation dose and for 10 kGy the level of o-tyrosine was equal to 1 -8%.