The dye-binding capacity (DBC) of basic groups in the proteins of test materials for Acid Orange 12 is measured before and after reaction with propionic anhydride to block the lysine groups. The difference in moles dye bound as a result of the reaction agrees well with the lysine moles present, as determined with fluoridinitrobenzene and by ion-exchange chromatography, for over 50 samples-fish, meat, milk, barley, peanut, grain legumes etc. The procedure takes approximately 40 min using commercial dye-binding equipment and longer using normal laboratory apparatus. It appears suitable for plant breeding studies and for quality control of damage to lysine in processed foods.