1. Beef thyroid preparations were tested for DPNH oxidase activity in a system supplemented with Mn 2+, dichlorophenol and sulfite ions and for the iodination of tyrosine in a system supplemented with either glucose and glucose oxidase or FMN and Mn 2+. 2. A soluble fraction was found to exhibit DPNH oxidase activity and tyrosine iodinating activity following removal of endogenous inhibitors by dialysis. 3. A particulate fraction was found to exhibit DPNH oxidase activity and tyrosine iodinating activity following removal of endogenous inhibitors by centrifugation. 4. Low molecular weight inhibitors present in the soluble fraction could be removed by dialysis or colected by ultrafiltration. Evidence is presented which suggests that ascorbic acid and GSH are among the inhibitors present in the soluble fraction. Other substances found to inhibit the iodination of tyrosine by the thyroid particulate fraction are DPNH, TPNH, cysteine, epinephrine, norepinephrine, serotonin, ergothioneine, thiohistidine and dichlorophenol. 5. The presence of peroxidase activity in the thyroid particulate fraction, which was suggested by the oxidation of DPNH in the presence of the supplements indicated above, was confirmed. An enzyme solubilized with digitonin and trypsin, and concentrated with ammonium sulfate was found to catalyze the oxidation of o-dianisidine, scopoletin and reduced 2,6-dichlorobenzeneindo-3′-chlorophenol by H 2O 2. 6. Purified preparations of myeloperoxidase and lactoperoxidase were found to catalyze the iodination of tyrosine in the presence of (a) glucose and glucose oxidase, (b) FMN and Mn 2+, (c) Mn 2+ plus a DPNH generating system or (d) Mn 2+ plus a TPNH generating system. 7. These results are discussed in relation to the mechanism of the biosynthesis of iodotyrosines.