Comparative study of iodination of chymotrypsin and trypsin was carried out using either iodine monochloride or the electrochemical procedure under conditions of controlled anode potential. It was shown that a part of iodine monochloride was consumed for oxidation of protein molecules. As a result of this process specific activity of the enzymes was decreased down to 50-60% of the initial activity after introduction of 2 atoms of iodine per a molecule. The similar degree of iodination under mild conditions of electrolysis in potentiostatic experiments caused considerably lower decrease in the enzymatic activity, reaching 5-10% of the initial activity (after introduction of I atom of iodine per a molecule--2.5%); labelling of trypsin and chymotrypsin by means of iodine radionuclides is promising for various medico-biological purposes.