The effect of the degree of iodination on the enzymatic activity of two enzymes, phospholipase A2 (fraction k1) and cathepsin D, was studied. The compounds were labelled with131I3 and131ICI 2– prepared by electrolysis at a controlled electrode potential. To avoid excessive denaturation of the enzyme, the electrophilic reagent was produced separately and added to the enzyme solution. Unreacted iodine species were removed by means of gel chromatography on Sephadex G-25. Enzymes were also radioiodinated by a chemical method, using chloramine T as oxidant, the results being compared with those obtained by the electrochemical method.