3,5-Diiodo-L-tyrosine (DIT) was immobilized on bead cellulose activated by the reaction with 2,4,6-trichloro-1,3,5-triazine (TCT). The amount of immobilized DIT depends on the TCT/DIT ratio in the reaction mixture up to the value 8 mol/mol, at higher values it becomes TCT/DIT-independent. Various sorbents containing 5-31 mu mol DIT/ml cellulose were prepared by varying the amount of DIT in the reaction mixture. The immobilization is also affected by pH. More DIT was bound in acid (pH 6.0) or neutral medium in comparison with the alkaline reaction conditions. Most of the experiments were performed at pH 9.1 equal to the ionization constant of NH2 group of L-tyrosine. The DIT/bead cellulose materials were prepared for use as sorbents in affinity chromatography of proteases.