o-Tyrosine has been proposed as a marker for postirradiation identification of food that contains protein. In this study, the validity of using o-tyrosine for this purpose has been tested and established. The validation process involved examination of background levels of o-tyrosine in unirradiated chicken, radiation dose yield, postirradiation storage, dose rate, radiation type, temperature during irradiation, and oxygen concentration during irradiation. o-Tyrosine is present in unirradiated chicken meat at variable levels. However, these background levels are low enough that o-tyrosine can serve to determine whether chicken has been irradiated or not at the commercially approved doses (3 kGy). The radiation dose response curve for the formation of o-tyrosine is linear. The apparent yields may vary with the analytical method used; however, it is independent of the dose rate, radiation type, atmosphere, and temperature (above freezing) during irradiation. It is also independent of the storage time and temperature after irradiation. It is concluded that this marker can be used to determine the absorbed dose in chicken meat irradiated with either gamma rays or electrons under normal or modified atmosphere.