We developed a histochemical method to demonstrate iodide peroxidase activity in various thyroid disorders and compared it with the biochemical and ultrastructural-cytochemical methods. All of the 26 adenomatous goiters and 43 follicular adenomas were peroxidase-positive. In the 74 cases of thyroid carcinomas examined, about half of the follicular carcinomas (17 of 33 patients), and a few papillary carcinomas (3 of 41 patients) were peroxidase-positive. Peroxidase-negative cases were seen in 70% (52 of 74 patients) of the follicular and papillary carcinomas. All the non-tumorous thyroid tissues adjacent to various disorders were peroxidase-positive. Since our histochemically demonstrated peroxidase activities almost parallel those determined biochemically and ultrastructural-cytochemically, we conclude that histochemical examination is a simple and useful method for the detection of peroxidase activity. As for the relationships between histochemically proved peroxidase activity and the histology of tumors, the histological differentiation of tumors was not consistent with their functional differentiation classified according to peroxidase activity.