the effects of iodide, thiocyanate and perchlorate, three anions with the same molecular size, on the oxidation of tyrosine to 3,3'-bityrosine by several peroxidases were evaluated at pH 8.8, i.e. in conditions in which iodide is not oxidized. The following results were obtained: 1. Iodide greatly stimulates the rate of bityrosine formation in the presence of thyroid peroxidase. No effect was seen with horseradish peroxidase or lactoperoxidase. Maximal iodide effects were obtained with about 0.5 mM iodide and Km for iodide was equal to about 0.028 mM. These results suggest that thyroid peroxidase contains a simple class of regulatory binding sites for iodide. 2. SCN- mimics iodide effects; maximal stimulatory effects were seen with about 0.5 mM thiocyanate and Km for SCN- was equal to 0.1 mM. The effects of SCN- and those of iodide were not additive. These results suggest that SCN- binds to the same regulatory site as iodide but with a slightly lower affinity. No effect of SCN- was seen with horseradish peroxidase or lactoperoxidase. 3. ClO-4, another anion with the same molecular size as iodide and SCN-, had neither an effect on the oxidation of tyrosine to bityrosine nor did it prevent the stimulatory effect of iodide on this reaction. Bromide was without effect on the same reaction.