THE finding of Schachner et al.1 that the in vitro formation of diiodotyrosine and thyroxine by thyroid slices, with radioactive iodine, 131I, as indicator, is inhibited by cyanide, azide, sulphide and carbon monoxide (particularly in the dark) led to an investigation of the occurrence of oxidases in the thyroid. The production of a strong peroxidase reaction and the recent publication of a paper by Dempsey2 focused attention on this enzyme. Dempsey, using the benzidine histological technique, reported the presence of peroxidase in the follicular cells of the thyroid. He claims that the peroxidase reaction is not due to haemoglobin, since it was discrete and there was no continuity between erythrocytes and intracellular reacting granules. The cellular peroxidase reaction was found to be inhibited by thiouracil and in much smaller concentration than was necessary to inhibit the haemoglobin reaction. Owing to the non-specificity of the benzidine reaction and the fact that the high concentration of hydrogen peroxide employed in the histological technique would effectively inhibit true peroxidase, however, this finding requires more careful investigation, because of the obvious implications of the reported inhibition by thiouracil.